A Trial to See if the Combination of Fianlimab With Cemiplimab Works Better Than Pembrolizumab for Preventing or Delaying Melanoma From Coming Back After it Has Been Removed With Surgery

This study is researching an experimental drug called REGN3767, also known as fianlimab (R3767), when combined with another medication called cemiplimab (each individually called a "study drug" or called "study drugs" when combined) compared with an approved medication called pembrolizumab. The objective of this study is to see if the combination of fianlimab and cemiplimab is an effective treatment compared to pembrolizumab in patients that have had melanoma removal surgery but are still at high risk for the recurrence of the disease. Pembrolizumab is an approved treatment in some countries in this clinical setting. The study is looking at several other research questions, including: * What side effects may happen from receiving the study drugs. * How much study drug is in the blood at different times. * Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects). Antibodies are proteins that are naturally found in the blood stream that fight infections. * How administering the study drugs might improve quality of life.

Key Inclusion Criteria: 1. All patients must be either stage IIB, IIC, III, or stage IV per American Joint Committee on Cancer (AJCC) 8th edition and have histologically confirmed melanoma that is completely surgically resected in order to be eligible as defined by the protocol 2. Complete surgical resection must be performed within 12 weeks prior to randomization, and enrollment may occur only after satisfactory wound healing from the surgery 3. All patients must have disease-free status documented by a complete physical examination and imaging studies within 4 weeks prior to randomization, as described in the protocol Key Exclusion Criteria: 1. Uveal melanoma 2. Any evidence of residual disease after surgery by imaging, pathology, or cytology. 3. Ongoing or recent (within 2 years) evidence of clinically significant autoimmune disease that required treatment 4. Uncontrolled infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C (HCV) infection; or diagnosis of immunodeficiency that is related to, or results in chronic infection, as described in the protocol 5. Another malignancy that is currently progressing or that required active treatment in the past 5 years, as described in the protocol 6. Participants with a history of myocarditis 7. Adolescent patients (≥12 to \<18 years old) with body weight \<40 kg Note: Other Protocol Defined Inclusion/ Exclusion Criteria Apply