Maintenance With Selinexor/Placebo After Combination Chemotherapy in Participants With Endometrial Cancer [SIENDO]

Inclusion Criteria:

* Female, at least 18 years of age at the time of informed consent.
* Histological confirmed endometrial cancer of the endometrioid, serous, or undifferentiated type. Carcinosarcoma of the uterus is also allowed.
* Completed a single line of at least 12 weeks of taxane-platinum combination therapy (not including adjuvant or neoadjuvant therapy), and achieved partial remission (PR) or complete remission (CR) according to RECIST version 1.1 for:
* Primary Stage IV disease, defined as:
* had a primary or later debulking surgery during first-line taxane-platinum therapy with R0 resection (R0 resection indicates a macroscopic complete resection of all visible tumor) and achieved CR after at least 12 weeks taxane-platinum chemotherapy, OR
* had a primary or later debulking surgery during first-line taxane-platinum therapy with R1 resection (R1 resection indicates incomplete removal of all macroscopic disease,) and achieved PR or CR after at least 12 weeks taxane-platinum chemotherapy, OR
* had no surgery and achieved PR or CR after at least 12 weeks taxane-platinum chemotherapy.

OR

* At first relapse (i.e., relapse after primary therapy including surgery and/or chemotherapy therapy for Stage I-IV disease), defined as:
* had Stage I-III disease at diagnosis and received at initial diagnosis adjuvant chemotherapy and relapsed later. Participants should have PR or CR after at least 12 weeks of taxane-platinum chemotherapy compared with the start of this chemotherapy at the time of relapse, OR
* had Stage I-III disease at diagnosis and did not receive adjuvant chemotherapy at initial diagnosis and relapsed later. Participants should have PR or CR after at least 12 weeks of taxane-platinum chemotherapy compared with the start of this chemotherapy at the time of relapse, OR
* had Stage IV disease at diagnosis and received initially chemotherapy with or without surgery and relapsed later. At the time of relapse, participants should have PR or CR after at least 12 weeks of taxane-platinum chemotherapy compared with the start of this chemotherapy at the time of relapse.

Participants that required their chemotherapy dose held during the 12-week therapy may be considered if they meet the other criteria above and achieve PR or CR per RECIST V1.1.

* Must be able to initiate study drug 5 to 8 weeks after completion of their final dose of chemotherapy.
* Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.
* Participants must have adequate bone marrow function and organ function within 2 weeks before starting study drug as defined by the following laboratory criteria:
* Hepatic function: total bilirubin up to 1.5\*upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) less than or equal to (≤) 2.5\*ULN in participants without liver metastasis. For participants with known liver involvement of their tumor: AST and ALT ≤5\*ULN.
* Hematopoetic function: Absolute neutrophil count (ANC) greater than or equal to (≥) 1.5\*10\^9/L; platelet count ≥100\*10\^9 per liter (/L); hemoglobin ≥9.0 gram per deciliter (g/dL).
* Renal function: estimated creatinine clearance (CrCl) of ≥20 milliliter per minute (mL/min), calculated using the Cockroft Gault formula.
* In the opinion of the Investigator, the participant must:
* Have a life expectancy of at least 12 weeks, and
* Be fit to receive experimental therapy.
* Premenopausal females of childbearing potential must have a negative pregnancy test (serum β-human chorionic gonadotropin test) prior to the first dose of study drug. Female participants of childbearing potential must agree to use highly effective methods of contraception throughout the study and for 1 week following the last dose of study drug.
* Written informed consent in accordance with federal, local, and institutional guidelines. The participant must provide informed consent prior to the first Screening procedure.

Exclusion Criteria:

* Has any sarcomas, small cell carcinoma with neuroendocrine differentiation, or clear cell carcinomas.
* Received a blood or platelet transfusion during 4 weeks prior to randomization.
* Being treated with a concurrent cancer therapy.
* Previous treatment with an exportin 1 (XPO1) inhibitor.
* Previous treatment with anti- programmed cell death protein 1 (PD-1) or anti-programmed cell death ligand-1 (PD-L1) immunotherapy (e.g., pembrolizumab).
* Concurrent treatment with an investigational agent or participation in another clinical trial.
* Participants who received any systemic anticancer therapy including investigational agents or radiation ≤3 weeks (or ≤5 half-lives of the drug \[whichever is shorter\]) prior to cycle 1 day 1 (C1D1). Palliative radiotherapy may be permitted for symptomatic control of pain from bone metastases in extremities, provided that the radiotherapy does not involve target lesions, and the reason for the radiotherapy does not reflect progressive disease (PD).
* Major injuries or surgery within 14 days prior to C1D1 and/or planned surgery during the on-treatment study period.
* Previous malignant disease, except participants with other malignant disease, for which the participant has been disease-free for at least 3 years. Concurrent other malignant disease except for curatively treated carcinoma in situ of the cervix or basal cell carcinoma of the skin.
* Any life-threatening illness, medical condition or organ system dysfunction, which, in the investigator's opinion, could compromise the participant's safety or compliance with the protocol.
* Known contraindications to selinexor.
* Known uncontrolled hypersensitivity to the investigational drug, or to its excipients.
* Radiotherapy to the target lesion within the past 3 months prior to baseline imaging.
* Persistent Grade 3 or 4 toxicity from previous chemotherapy and/or radiotherapy, with the exception of alopecia.
* Active brain metastases (e.g., stable for \<8 weeks, no adequate previous treatment with radiotherapy and/or surgery, symptomatic, requiring treatment with anti-convulsants. Corticoid therapy is allowed if administered as stable dose for at least 1 month before randomization).
* Known unstable cardiovascular function:
* Symptomatic ischemia, or
* Uncontrolled clinically significant conduction abnormalities (i.e., ventricular tachycardia on anti-arrhythmia are excluded; 1st degree atrioventricular block or asymptomatic left anterior fascicular block /right bundle branch block will not be excluded), or
* Congestive heart failure of New York Heart Association Class ≥3, or
* Myocardial infarction within 3 months
* Females who are pregnant or actively breastfeeding.
* Uncontrolled (i.e., clinically unstable) infection requiring parenteral antibiotics, antivirals, or antifungals within 1 week prior to first dose; however, prophylactic use of these agents is acceptable even if parenteral.
* Active hepatitis C and/or B infection.
* Participants unable to swallow tablets, participants with malabsorption syndrome, or any other gastrointestinal (GI) disease or GI dysfunction that could interfere with absorption of study drug. A history of bowel obstruction requiring a nasogastric tube or intravenous infusion during the past 2 months is not allowed (except when this obstruction is caused by surgery or other non-malignant causes).
* Psychiatric illness or substance use that would prevent the participant from giving informed consent or being compliant with the study procedures.
* Participants unwilling or unable to comply with the protocol.
* Persons who have been committed to an institution by official or judicial order.
* Participants with dependency on the Sponsor, Investigator or study site.