Custom hook : post-update when merge request


I have a gitlab server and when I push a modification from my local remote, I have a custom hook on my gitlab server (post-update) who is running. And in this hook, I have a command to deploy my application, if my branch is “development”.
But when I use gitlab to merge two branch, the “post-update” hook don’t fire, I have just “update” but I need the “post-update” to deploy after a merge request.
I have test post-merge but it’s not fire too.

Have you got an idea, without using webhooks ?