The purpose of the study is to understand how the study medicine PF-06823859 (dazukibart) works in people with idiopathic inflammatory myopathies (DM and PM). These disorders cause inflammation that weakens the muscles that are important for movement and may also cause skin rash in people with DM. This study is seeking participants who: * Are 18 years of age or older or minimum legal adult age as defined per local regulation, whichever is greater * Have active DM or active PM. * Are receiving a stable dose of 1 corticosteroid taken by mouth and/or 1 traditional immunosuppressant. * Note: Corticosteroids and immunosuppressants are medicines that help reduce inflammation and may signal to the immune system not to attack the body. Dermatomyositis (DM) is a rare disease that causes muscle inflammation that results in muscle weakness and low muscle stamina. Patients with DM have a characteristic skin rash. Polymyositis (PM) is a rare disease that involves mainly muscle inflammation resulting in muscle weakness, that can sometimes be painful. Patients with DM and PM may have trouble going up the steps, walking or getting to a standing position. Some of the participants will receive the study medicine (dazukibart) and some will receive placebo (which is similar to study medicine but contains no medicine in it). The study medicine or placebo will be given as an intravenous (IV) infusion (directly into the veins), which takes about 1 hour; every 4 weeks from Day 1 to Week 48 of the study. Both dazukibart and placebo and will be given at the study site. The study will compare the experiences of people receiving study medication to those of the people who do not. This will help to see if dazukibart is safe and effective. Participants will take part in this study for about 13 months. During this time, participants will have 15 study visits. These visits will be performed at the study site.