A Study to Find Out if Selexipag is Effective and Safe in Patients With Chronic Thromboembolic Pulmonary Hypertension When the Disease is Inoperable or Persistent/Recurrent After Surgery and/or Interventional Treatment

Main Inclusion Criteria: * Signed and dated informed consent form * Male and female participants from greater than or equal to (\>) 18 (or the legal age of consent in the jurisdiction in which the study is taking place) and less then or equal to (\<=85) years old at Screening (Visit 1) * With established diagnosis of inoperable CTEPH (i.e., technically non-operable) or persistent/recurrent CTEPH after pulmonary endarterectomy (PEA) and/or balloon pulmonary angioplasty (BPA), as confirmed by the corresponding adjudication committee * With pulmonary hypertension (PH) in WHO FC I-IV. * Participant able to perform the 6-minute walk test (6MWT) with a minimum distance of 100 m and a maximum distance of 450 m at screening visit. * Women of childbearing potential must have a negative pregnancy test at screening and randomization and must agree to undertake monthly urine pregnancy tests, and to use a reliable method of birth control from screening visit up to at least 30 days after study treatment discontinuation. If a hormonal contraceptive is chosen it must be taken for at least 1 month prior to randomization. Main Exclusion Criteria: * Planned or current treatment with another investigational treatment up to 3 months prior to randomization. * Any known factor or disease that might interfere with treatment compliance, study conduct, or interpretation of the results, such as drug or alcohol dependence or psychiatric disease. * Known concomitant life-threatening disease with a lif…