pycharm refusing to merge unrelated histories with the UI of merge conflicts

Me and my friend developed an app, each of us used different repository(yes..  Not very smart but that's the case)...

 We wish to merge our different files and conflicts using the convenient pycharm merge UI, the problem is that when trying to merge we get the error " refusing to merge unrelated histories", because each of us used a different repository.

We can easily solve this issue using the flag --allow-unrelated-histories in the command line:

git merge --alow-unrelated-histories master

Which works but let us handle the conflicts one by one in text editor without the convenient UI merging pycharm offers...

How do I add the flag --alow-unrelated-histories to the merge command while using the GUI interface?