A Study to Evaluate Single Agent Selinexor Versus Physician's Choice in Participants With Previously Treated Myelofibrosis

Inclusion Criteria: * A diagnosis of primary MF or post-essential thrombocythemia (ET) or post-polycythemia (PV) MF according to the 2016 World Health Organization (WHO) classification of myeloproliferative neoplasms (MPN), by the most recent local pathology report. * Previous treatment with JAK inhibitors for at least 6 months. * Measurable splenomegaly during the screening period as demonstrated by spleen volume of ≥450 centimeter cube (cm\^3) by magnetic resonance imaging (MRI) or computerized tomography (CT) scan. * Relapsed, Refractory or Intolerant to JAK inhibitors as defined as meeting one of the criteria below: * less than (\<) 35% spleen volume reduction by MRI or CT-scan (from baseline) or * \<50% decrease in spleen size by palpation (from baseline) or an increase of at least 3 cm with the spleen at least 5 cm below the left costal margin or * Spleen volume increase greater than (\>) 25% from nadir or a return to within 10% of baseline after any initial response or * Treatment with JAK inhibitor was complicated by development of red blood cells (RBC) transfusion requirement (2 units per month for 2 month); or grade 3 thrombocytopenia, anemia, hematoma/hemorrhage; or grade 2 non-hematologic toxicity while on JAK inhibitors * Participants ≥18 years of age. * Eastern Cooperative Oncology Group (ECOG) less than or equal to (≤) 2. * Platelet count ≥75\*10\^9 per liter (/L). * Absolute neutrophil count (ANC) ≥1.5\*10\^9/L. * Serum direct bilirubin ≤1.5\*upper l…