The purpose of this study is to evaluate whether elranatamab monotherapy can provide clinical benefit compared to lenalidomide monotherapy (control) in participants with newly diagnosed multiple myeloma after undergoing autologous stem cell transplant. In Part 1 and Part 2 of the study, participants in the study will either receive elranatamab (arm A and C) as an injection under the skin at the study clinic or lenalidomide orally once daily at home (arm B). Participation in the study will be approximately five years