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 mofetil or sodium, azathioprine, leflunomide, cyclosporine, oral tacrolimus) for DM or PM * Participants have a body mass index (BMI) lower or Equal to 40.0 kilograms per square meter (kg/m\^2) * Other protocol defined inclusion criteria could apply Exclusion Criteria: * Primary diagnosis of inclusion body myositis (IBM), malignancy-associated myositis (defined as diagnosis of myositis within 3 years of cancer), immune mediated necrotizing myopathy (IMNM) with a biopsy characterized as necrotizing biopsy or IMNM with positive anti-signal recognition particle antibody (SRP) or anti 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) auto antibodies. Participants with anti-transcription intermediary factor 1 (TIF1) gamma antibody or newly diagnosed (within 1 year) anti MDAT5 antibody should have had adequate screening for cancer within 12 months of Day 1. Adequate screening of cancer is defined as up-to-date age and gender appropriate screening as per national guidelines * Primary diagnosis of juvenile DM, or adult participants previously diagnosed with juvenile DM * Any other active concurrent connective tissue disease associated with inflammatory myopathy in the Investigator's opinion. Eligibility of participants with diagnosis of concurrent connective tissue disease(s) will be reviewed and approved by an idiopathic inflammatory myopathies (IIM) expert committee * Severe interstitial lung disease defined as supplemental oxygen required at rest, or forced vital capacity (FVC) of \<60 percent (%) predicted. Participants within 1 year of PM/DM diagnosis and anti-MDA5 antibody, should have been evaluated for interstitial lung disease (ILD) with high resolution computed tomography (HRCT) Chest * Any uncontrolled disease (for example \[e.g.\], severe respiratory, cardiovascular, gastrointestinal, neurological, psychiatric, hematological, metabolic \[including thyroiditis with increased/decreased thyroid stimulating hormone (TSH)\], renal \[Estimated glomerular filtration rate \< 40 milliliter per minute/1.73 m\^2 as calculated by the Modification of Diet in Renal Disease equation by the central laboratory\], hepatic, endocrine/reproductive organ disease) other than DM/PM, that in the Investigator's or Sponsor/designee's opinion constitutes an inappropriate risk or contraindication for participation in the study or that could interfere with the study objectives, conduct, or evaluation * Other protocol defined exclusion criteria could apply