Excuse me, I don't really know how to do this.
Should I create a php file like DifferentialJIRAIssuesCommitMessageField.php?
And Differential would treat some pridicular string as a custom field.
Then how do I add parsing external bug tracker id code?

Extend a Remarkup rule if you want to parse a specific URL pattern into a token like we do for tasks like T5000.

Doorkeeper is the correct upstream project to follow for two-way integration in the long run, it's just not on any roadmap currently. You can extend Phabricator yourself today for these things if needed.

Reference: Adding LaTeX support into the Phabricator team collaboration system
Should I extend PhabricatorRemarkupCustomInlineRule also?

Could you kindly suggest me extend which Remarkup rule for parsing a specific URL pattern into a token like we do for tasks like T5000.
I can't find where you do this exactly.