This is a Phase 2, multicenter, two-arm, open-label study to evaluate the safety and efficacy of selinexor versus treatment per physician's choice (PC) in participants with myelofibrosis (MF) who had at least 6 months of treatment with a Janus kinase (JAK)1/2 inhibitor. Study participants will be randomized in a 1:1 ratio to either receive selinexor or physicians' choice of treatment.