Pull request template for Bitbucket
Bitbucket Cloud (BitBucket.org)
Such a feature was not supported, but now is! It's called Default Description, and it's been available since Nov 2018 if you use bitbucket.org.
You can access it in your repository's settings page.
Bitbucket Server
If you use your own instance of BitBucket, there's an equivalent feature in the works: BSERV-8663.
Unfortunately, pull request template is not supported for Atlassian Bitbucket recently. And the feature has not been added in their roadmap so far.
More details, you can also refer the issue Custom Pull Request Description Template (BB-14637) and the post Creating a template for pull request.