Currently it is not possible to provide main branch during repository creation or update the repository's main branch later using the API endpoint /2.0/repositories/{workspace}/{repo_slug} 

API documentation: https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Bworkspace%7D/%7Brepo_slug%7D#put

Please update the API endpoint to support mainbranch update.