The purpose of this study is to evaluate pembrolizumab (MK-3475) subcutaneous (SC) administration as the first-line therapy in the treatment of metastatic squamous and nonsquamous NSCLC by assessing the pharmacokinetics (PK), safety, and efficacy of pembrolizumab SC injection in combination with standard-of-care chemotherapy. The primary hypothesis of the study is Pembrolizumab SC is noninferior to pembrolizumab intravenous (IV) for Cycle 1 Area Under Curve (AUC) and Cycle 6 minimal concentration (Ctrough) at steady state. Participants who discontinue study treatment after receiving the first course of 35 administrations of pembrolizumab (approximately up to 2 years) for reasons other than disease progression or intolerability, may be eligible for a second course of pembrolizumab for up to approximately 1 additional year if they have experienced radiographic disease progression per RECIST 1.1 as assessed by BICR after stopping first course treatment.