Study of Oral Atogepant When Added to OnabotulinumtoxinA (BOTOX) to Assess Adverse Events and Change in Disease Activity in Adult Participants With Chronic Migraine

Inclusion Criteria: * At least a 1-year history of chronic migraine (CM), with or without aura, consistent with a diagnosis according to International Classification of Headache Disorders 3rd edition (ICHD-3 2018) and with or without acute medication overuse as defined in the protocol. * Must be currently treated with BOTOX for CM: treated with \>= 2 treatment cycles in the 8 months prior to Visit 2 (Day 1) with documentation of payer authorization or written attestation of self-pay to support continued use of BOTOX. * Must have 8 to 23 (inclusive) migraine days in the electronic diary \[eDiary\] screening/baseline period (eDiary data must have been collected for at least 20 days). Exclusion Criteria: * Use of opioid-containing products for more than 4 days per month for acute treatment of headache in the 3 months prior to Screening or during the screening/baseline period. * Treatment of study target muscles using acupuncture, transcutaneous electrical nerve stimulation (TENS), cranial traction, dental splints for headache, or head and/or neck injections of anesthetics/steroids within 4 weeks prior to Screening and throughout the study. * Concurrent use of any migraine prevention treatment other than BOTOX (required concomitant medication; or topiramate \<=100mg daily) including use of oral gepants in the 4 weeks prior to screening nor during the screening/baseline period. * Current use or use within the 6 months (24 weeks) prior to Screening, of mAbs blocking the CGRP pat…