Migraine is characterized by attacks of throbbing, moderate or severe headache, often associated with nausea, vomiting, and/or sensitivity to light and/or sound. Chronic migraine, which occurs in 1-2 % of the population is characterized by 15 or more headache days/month for more than 3 months and at least 8 days/month with features of migraine headache. The study will evaluate the efficacy of onabotulinumtoxin A when added to CGRP monoclonal antibody therapy in chronic migraine prevention. Adverse events and change in disease activity will be monitored. Onabotulinumtoxin A and CGRP monoclonal antibody therapy are investigational drugs developed to prevent chronic migraine. Approximately 450 patients will be included from sites in Norway. All participants will receive CGRP monoclonal antibody therapy. Additionally, the participants will be randomized to receive onabotulinumtoxin A or placebo injections. Total study duration is 20 weeks including 3 on site visits and 3 telephone visits. After an inclusion visit the participants are registering data in an electronic headache diary using the application Brain Twin for a minimum of 4 weeks before the come to the randomization visit and the study medications are started. The duration of treatment is 12 weeks.