The purpose of the phase 1 part of this study is to evaluate how well pirtobrutinib is tolerated and what side effects may occur. The phase 2 part of the study will further investigate efficacy and safety of multiple pirtobrutinib dosages versus placebo. The study drug will be administered orally in participants with Primary Immune Thrombocytopenia (ITP). Blood tests will be performed to check how much pirtobrutinib gets into the bloodstream and how long it takes the body to eliminate it. The study will last up to approximately 16 weeks for phase 1 dose-escalation and 28 weeks for phase 2 dose-optimization, excluding screening.