There is currently no way to register a build trigger in Bitbucket using a multibranch pipeline project. A UI trigger should be added to the configuration page in the same manner as Freestyle and Pipeline projects.

Workaround:

Until this feature is added, you can still create a webhook in your Bitbucket Server instance manually:

• Go to your Bitbucket instance, and navigate to the affected repository
• Go to Settings -> Webhooks -> create
• Fill in the name and base URL for your Jenkins instance
• Check the Repository "Push" event