Locate the section for your Stash remote in the .git/config file. It looks like this:
[remote "origin"]
	fetch = +refs/heads/*:refs/remotes/origin/*
	url = https://your.stash.domain/stash/scm/project/repo.git
Now add the line fetch = +refs/pull-requests/*/from:refs/remotes/origin/pr/* to this section. Obviously, change the Stash url to match your project's URL. It ends up looking like this: