Connect issues to merge requests

I know that I have at least two ways to connect issue to an MR:

• click "New Branch" for issue and create an MR for it
• have something like "Fix hashtag number" in commit message

This will connect issue and MR and when I accept MR, those issues are closed.

But what if I have already resolved issues but forgot to say "Fix ..."? What if I want to manually specify which issues I want to close when accepting MR?