Merge request URL from CI MERGE_REQUEST environment variables

Just saw the only: [merge_requests] pipeline specifier and wanting to be able to create a url to the merge request from the gitlab environment variables so I can pass it into a deployment environment. I was hoping to be able to pass just an id, like CI_MERGE_REQUEST_ID, but that is not the id used in urls. Is there anyway to generate the merge request from one of the CI environment variables?