This is a global, multicenter, 2-part study to evaluate the efficacy and safety of selinexor plus ruxolitinib in JAK inhibitor (JAKi) treatment-naïve myelofibrosis (MF) participants. The study will be conducted in two phases: Phase 1 (open-label) and Phase 3 (double-blind). Phase 1 (enrollment completed) was an open-label evaluation of the safety and recommended Phase 2 dose (RP2D) of selinexor in combination with ruxolitinib and included a dose escalation using a standard 3+3 design (Phase 1a) and a dose expansion part (Phase 1b). Phase 3 (ongoing), double-blind, placebo-controlled part of the study comparing the efficacy and safety of combination therapy of selinexor + ruxolitinib with combination of placebo + ruxolitinib.