How can I reference JIRA issue number in the merge request template?

In a custom merge request template, how can I add a placeholder for the related JIRA issue?

The default merge request description while creating a merge request seems to recognize the JIRA number from the commit or the branch name. The message is “Closes DLP-488” or whatever JIRA issue you reference.

I would like to use this feature in my custom merge request template. In the markdown merge request template, I unsuccessfully tried things like:

#issue
<issue>