Custom button in Gitlab

Is possible to add a custom button to a merge request? I am looking for a similar button to the merge button. For example, the custom button could be useful if we wanted to merge to a staging branch in one click before merging to master.

GUI customisation isn’t possible as far as I am aware.

Thank you for your reply! is there any way that maybe with webhooks ? once a pull request is merged redirect to a page with a form (so I can get that data and send it to another app)? I am really new to gitlab

You could check the Gitlab API documentation to see if there is anything there that might help. See here: REST API resources | GitLab