Study of M5049 in DM and PM Participants (NEPTUNIA)

Inclusion Criteria: * Diagnosis of probable or definite DM or PM as per 2017 ACR/EULAR classification criteria, with positive autoantibody status. Anti-synthetase syndrome (ASyS) participants that meet classification criteria are allowed * Active disease on standard of care (SoC), must meet 1 of the criteria within 6 months prior to Screening: Pathological evidence of active myositis in muscle biopsy; Evidence of active myositis by Electromyography (EMG); Magnetic resonance imaging (MRI) with evidence of active myositis; or any muscle enzyme greater than or equal to (\>=) 4 × upper limit of normal (ULN) at time of Screening; Active PM/DM skin rash as per cutaneous dermatomyositis area and severity index-A (CDASI-A) \>= 7 at time of Screening * Minimum disease severity defined by: moderate to severe myopathy with manual muscle testing-8 (MMT-8) \>= 80 and less than or equal to (\<=) 142 AND at least 2 of the following core set measures (CSM) abnormalities: Patient Global Activity (PtGA) \>= 2 centimeters (cm); Physician Global Activity (PGA) derived from myositis disease activity assessment tool (MDAAT) \>= 2 cm; Extramuscular Activity Assessment derived from MDAAT \>2 cm; At least 1 muscle enzyme \> 1.5 times ULN; health assessment questionnaire-disability index (HAQ-DI) \>= 0.25 * Stable doses of oral corticosteroids (CS) and/or maximum of 1 non-corticosteroid immunosuppressive/immunomodulatory medications (methotrexate, 6 mercaptopurine, sulfasalazine, mycophenolate mofeti…