Can I use an external issue tracker like Jira

Yes, you can use external issue trackers. If you enable external issue tracking, you can type issues like issue #1234 or ISS-1234 and they will automatically link to the URL provided in setup.

To setup external issue tracking, visit your repo settings, click on "Use External Issue Tracker" and fill out the URL fields and select Numberic (#1234) or Alphanumeric (ABC-123, ISSUE-8675).