The primary objective of the study is to compare disease-free survival (DFS) of patients with high-risk cutaneous squamous cell carcinoma (CSCC) treated with adjuvant cemiplimab, versus those treated with placebo, after surgery and radiation therapy (RT). The secondary objectives of the study are: * To compare the overall survival (OS) of high-risk CSCC patients treated with adjuvant cemiplimab, versus those treated with placebo, after surgery and RT * To compare the effect of adjuvant cemiplimab with that of placebo on patients' freedom from locoregional recurrence (FFLRR) after surgery and RT * To compare the effect of adjuvant cemiplimab with that of placebo on patients' freedom from distant recurrence (FFDR) after surgery and RT * To compare the effect of adjuvant cemiplimab with that of placebo on the cumulative incidence of second primary CSCC tumors (SPTs) after surgery and RT * To evaluate the safety of adjuvant cemiplimab and that of placebo in high-risk CSCC patients after surgery and RT * To assess cemiplimab pharmacokinetics and immunogenicity in human serum