A Study to Evaluate INCB099280 in Participants With Advanced Cutaneous Squamous Cell Carcinoma

Inclusion Criteria:

* Histopathological diagnosis of cSCC.
* Previously untreated or recurrent locally advanced (without nodal metastases) or metastatic (distant or regional metastasis) cSCC not amenable to curative surgery and/or radiotherapy.
* Measurable disease based on either radiographic imaging per RECIST 1.1 or WHO criteria.
* Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.
* Life expectancy \> 3 months.
* Willingness to avoid pregnancy or fathering children.

Exclusion Criteria:

* Known history of an additional malignancy.
* Central nervous system (CNS) metastases requiring treatment and/or leptomeningeal disease.
* Toxicity from prior therapy that has not recovered.
* Prior receipt of an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent; treatment with an immune modulator (eg, CTLA-4, GITR, LAG3, TIM3, OX40, ICOS, IL-2, 4-1BB, CAR-T cell).
* Received thoracic radiation within 6 months of the first dose of study treatment.
* Participation in another interventional clinical study while receiving INCB099280.
* Impaired cardiac function or clinically significant cardiac disease.
* History or evidence of interstitial lung disease including noninfectious pneumonitis.
* Presence of gastrointestinal conditions that may affect drug absorption.
* Any autoimmune disease requiring systemic treatment in the past 5 years.
* Diagnosis of primary immunodeficiency or receiving chronic systemic steroid therapy at a daily dose exceeding 10 mg of prednisone or equivalent.
* Active infection requiring systemic therapy.
* History of organ transplantation, including allogeneic stem cell transplantation.
* Receipt of systemic antibiotics within 28 days of first dose of study treatment.
* Probiotic usage is prohibited during screening and throughout the study treatment period.
* Received a live vaccine within 28 days of the planned start of study drug.
* Laboratory values outside the Protocol-defined ranges.
* Inadequate organ function.

Other protocol-defined Inclusion/Exclusion Criteria may apply.