A Clinical Study of Intismeran Autogene (V940) and Pembrolizumab (MK-3475) in People With Melanoma (V940-012/INTerpath-012)

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Has unresectable and histologically confirmed Stage III or IV cutaneous melanoma per American Joint Committee on Cancer (AJCC) Eighth Edition guidelines. * Has been untreated for melanoma except if participant received prior adjuvant or neoadjuvant therapy with targeted therapy or immunotherapy (such as anti-cytotoxic T-lymphocyte-associated protein \[CTLA-4\], anti-programmed cell death 1 protein \[PD-1\] therapy or interferon), and only if relapse did not occur within 12 months after treatment discontinuation. * Have documentation of serine/threonine-protein kinase B-raf (BRAF) V600-activating mutation status or had BRAF V600 mutation testing per local institutional standards during the screening period (participants with BRAF mutation positive melanoma as well as BRAF wild-type or unknown are eligible). * Have the presence of at least 1 measurable lesion by computed tomography (CT) or magnetic resonance imaging (MRI) per Response Criteria in Solid Tumors Version 1.1 (RECIST 1.1) as determined by the local site investigator/radiology assessment. * Provides tumor tissue (preferably from a metastatic site and, if not available, from the primary tumor) that is suitable for next generation sequencing and biomarker analysis as required for this study. * Participants with human immunodeficiency virus (HIV) must have well controlled HIV on antiretroviral therapy (ART). * Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to randomization. * Participants with history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable at screening. Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Has clinically significant heart failure, defined as New York Heart Association class III or IV, within the past 6 months, unless the disease is well controlled in the opinion of the investigator. * HIV-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease. * Has ocular or mucosal melanoma. * Received transfusion of blood products (including platelets or red blood cells) or administration of colony-stimulating factors (including granulocyte colony-stimulating factor, granulocyte macrophage colony-stimulating factor, or recombinant erythropoietin) within 2 weeks of the Screening blood sample (including the blood sample for V940 generation). * Received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent, or with an agent directed to another stimulatory or coinhibitory T-cell receptor (eg, CTLA-4, lymphocyte activation gene 3 \[LAG-3\], tumor necrosis factor receptors \[OX-40 or CD137\]), with some exceptions. * Received prior systemic anticancer therapy for melanoma before randomization, with some exceptions. * Received prior radiotherapy within 2 weeks of start of study intervention or has ongoing radiation related toxicities. * Received a live or live-attenuated vaccine within 30 days before the first dose of study intervention. * Received prior treatment with another universal or personalized cancer vaccine.