UI controls in issue details

How can I get my own controls in Issue Details view? The way GitLab app does this:

I’ve spent the whole day trying out different ways, both Forge and Connect, and I don’t understand how to do this.

Here’s the GitLab app on the marketplace just in case: GitLab for Jira Cloud | Atlassian Marketplace

OK, I found the " Development Tool" module that does this.