What does PR merge means?
Can you confirm you are using Atlassian Stash to manage your Git repo?
When you create the PR, Stash try a "lazy" merge (in the /merge refs ID). If the target branch is moving, Stash will attempt new merges.
Some explanation about this lazy merge here.
If you trigger a Jenkins build with a Stash hook, Stash will send all the PR information to Jenkins (pullRequestId, from SHA1, merge SHA1, ...).
So with your Jenkins job, you can try a merge (origin/pr/${pullRequestId}/from to origin/master, if master is your target branch) and you can build the merge result.
This what we are doing in my company with these settings and the Stash pullrequest builder plugin:
It's working well :)
Every time a developer updates the PR, a new PR build is triggered and Jenkins tries to do a new merge.
Once the PR is validated, if the developer click on the Merge button, it'll try to merge the code on the target branch.
You can set some merge options in this properties file.