This clinical trial is evaluating whether addition of navtemadlin to ruxolitinib treatment will provide more clinical benefit than ruxolitinib alone for patients with Myelofibrosis who have a suboptimal response to ruxolitinib treatment alone. Subjects will start by receiving ruxolitinib alone in the run-in period. Those who demostrate a suboptimal response from ruxolitinib alone will then be randomized 2:1 to receive navtemadlin or navtemadlin placebo as add-on treatment to their ongoing ruxolitinib. Randomized means that subjects will be assigned to a group by chance, like a flip of a coin. The study is blinded, meaning the subjects, doctors, central endpoint assessors and sponsor will not know which add on treatment (navtemadlin or navtemadlin placebo) the subject is receiving.