This is a multicenter, genetically-randomized, controlled, Phase 3 study evaluating the efficacy and safety of T-cell receptor-engineered donor T cells targeting HA-2 (TSC-101) administered following reduced-intensity conditioning (RIC) hematopoietic cell transplantation (HCT) in participants with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS). The study will compare TSC-101 plus standard of care (SOC) versus SOC alone in participants undergoing allogeneic peripheral blood stem cell transplantation from haploidentical or mismatched unrelated donors.