1. Create the token
At id.atlassian.com > Security > API tokens, create a scoped token with the repository:write and repository:issue:write scopes for the workspace. Atlassian has retired app passwords in favor of these scoped API tokens; an existing app password still works for now but is not what new connections should use.
2. Add the connection
Tracker Bitbucket Issues, repository as workspace/repo_slug, your Atlassian account email, and the token. The base URL stays empty (api.bitbucket.org); Bitbucket Cloud has no self-hosted variant to point at.
3. Test, then file
Test connection reads the repository and refuses one with the issue tracker turned off. Bitbucket's issue tracker has no label field, so the realuptime label plus your defaults are appended as a line in the ticket body instead of being dropped.
Go deeper
The full reference lives in the docs: API documentation. Error codes named above are each explained in the error-code reference.