There's the "Default Task" feature in Bitbucket, where you can create a Task that shows up on all created PRs. Except release branches, for some weird reason. I have no idea why that exemption exists.

https://www.atlassian.com/blog/bitbucket/default-pull-request-tasks

For the use case where you want to have tasks on PRs for release branches, you could write a script that creates a task on a PR through Bitbucket's API and call that script through in your pipeline.

https://community.atlassian.com/t5/Bitbucket-questions/How-to-create-Pull-Request-task-using-REST-api/qaq-p/720958