Trigger build when branch is merged

Hello,

I am facing a problem when I try to trigger a build on my develop branch.

I already trigger builds when merge requests are opened. It works fine.
Now, I would like to trigger a build of my target branch when a merge request is accepted.

In details :
* I create a feature branch
* I open a merge request for this feature
* Jenkins triggers a build of the feature branch
* I accept the merge request to merge my feature into my develop branch
* Jenkins does not triggers build of develop branch

How can I do to trigger a build of my develop branch when a feature branch is merged ?
Thanks in advance.

Regards

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: 12.6.4-ee
- **Jenkins version**: 2.205
- **Job type**: Pipeline