Study of Pembrolizumab (MK-3475) in Adults With Recurrent/Metastatic Cutaneous Squamous Cell Carcinoma (cSCC) or Locally Advanced Unresectable cSCC (MK-3475-629/KEYNOTE-629)

Inclusion Criteria: * R/M cSCC cohort only: * Has cSCC that is either metastatic defined as disseminated disease, and/or unresectable disease that is not curable by surgery or radiation. * Has histologically-confirmed cSCC as the primary site of malignancy (metastatic skin involvement from another primary cancer or from an unknown primary cancer is not permitted). * LA cSCC cohort only: * Must be ineligible for surgical resection. * Participants who received prior radiation therapy (RT) to index site or must be deemed to be not eligible for RT. * Participants who received prior systemic therapy for curative intent are eligible regardless of regimen. * R/M cSCC cohort only: * Has metastatic disease defined as disseminated disease distant to the initial/primary site of diagnosis, and/or must have locally recurrent disease that has been previously treated (with either surgery or radiotherapy), and is not amenable to either curative surgery or radiotherapy. * Has measurable disease based on RECIST 1.1 as assessed by the central imaging vendor. * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 within 10 days prior to the start of study treatment. * Has adequate organ function. * Has a tissue sample adequate for programmed death-ligand 1 (PD-L1) testing as determined by central laboratory testing prior to study allocation. * Has a life expectancy \>3 months. * Female participants of childbearing potential must agree to use an adequate method of contra…