A Study of Pembrolizumab (MK-3475) With or Without Lenvatinib (E7080/MK-7902) as First Line (1L) Intervention in a Programmed Cell Death-ligand 1 (PD-L1) Selected Population With Recurrent or Metastatic Head and Neck Squamous Cell Carcinoma (R/M HNSCC) (MK-7902-010) (KEYNOTE-010)

Inclusion Criteria: * Has histologically confirmed diagnosis of R/M HNSCC that is considered incurable by local therapies. Note: Participants with newly-diagnosed HNSCC must be M1/Stage IV. * Has a primary tumor location of oropharynx, oral cavity, hypopharynx, or larynx. Note: Primary tumor site of nasopharynx (any histology) or unknown primary tumor (including p16+ unknown primary) are not eligible. Contraceptive use by men should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. If the contraception requirements in the local label for any of the study interventions is more stringent than the requirements above, the local label requirements are to be followed. * Male participants agree to use approved contraception during the treatment period for at least 7 days after the last dose of lenvatinib/placebo, or refrain from heterosexual intercourse during this period * Female participants are not pregnant or breastfeeding, and are not a woman of childbearing potential (WOCBP), OR are a WOCBP that agrees to use contraception during the treatment period (or 14 days prior to the initiation of study treatment for oral contraception) and for at least 120 days post pembrolizumab, or 30 days post lenvatinib/placebo, whichever occurs last * Has measurable disease per RECIST 1.1 as assessed by BICR. Note: Lesions situated in a previously irradiated area are considered measurable if progression has been shown in…