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.