A Study to Evaluate the Safety, Tolerability, Efficacy, and Drug Levels of CC-97540 in Participants With Relapsing Forms of Multiple Sclerosis, Progressive Forms of Multiple Sclerosis or Refractory Myasthenia Gravis (MG) (Breakfree-2)

Inclusion Criteria \- Relapsing forms of Multiple Sclerosis (RMS) - Cohort 1. i) Participants must have an Expanded Disability Status Scale (EDSS) of ≥ 3.0 and ≤ 5.5. ii) Participants must have a diagnosis of Multiple Sclerosis (MS) with relapsed/refractory MS or conversion to active secondary progressive multiple sclerosis (aSPMS), and worsening of disease within 12 months prior to Screening and while on treatment with a high-efficacy DMT for at least 6 months. \- Progressive forms of MS - Cohort 2. i) Participants must have an EDSS ≥ 3.0 and ≤ 6.0. ii) Participants must have a diagnosis of primary progressive multiple sclerosis (PPMS) that is treatment-resistant or diagnosis of inactive secondary progressive multiple sclerosis (iSPMS). \- Myasthenia Gravis - Cohort 3 i)MGFA classification of II-IV at screening ii) Documentation of autoantibodies against AChR or MuSK (historical or at Screening) iii) Refractory disease defined as disease activity on at least 2 immunosuppressants, including steroids, NSIs, or biologics. iv) Has had thymectomy, only if indicated according to current guidelines. Exclusion Criteria - Cohorts 1 and 2: Participants that cannot complete the 9-Hole Peg Test (9-HPT) in at least 1 hand in \<240 seconds unless extenuating medical conditions unrelated to MS prohibit this. - Participants that cannot perform a Timed 25-Foot Walk Test (T25FWT) in \< 150 seconds. - Presence of other confounding peripheral nervous system disorders or other disorders that may impact muscle strength (eg, myositis) or cause weakness, stroke, chronic inflammatory demyelinating polyradiculoneuropathy, Lambert-Eaton myasthenic syndrome. - Other protocol-defined Inclusion/Exclusion criteria apply. Always talk to your doctor about whether this trial is right for you.