The purpose of this study is to learn if intismeran autogene which is an individualized neoantigen therapy (INT; formerly, called messenger ribonucleic acid \[mRNA\]-4157) with pembrolizumab (MK-3475) is safe and prevents cancer from returning in people with high-risk melanoma. Researchers want to know if intismeran autogene with pembrolizumab is better than receiving pembrolizumab alone at preventing the cancer from returning.