Why don’t you want to rely on YunoHost’s permission system?
Edit: to be tested, and may break your app. If so, alter manually the file back to its previous state:
with the command nano /home/yunohost.app/radarr/config.xml try changing UrlBase to <UrlBase>mydomain.com/radarr</UrlBase>. Restart Radarr service afterwards.
Then it is most likely an issue with Radarr. You are most welcome to report the bug in their repository. I unfortunately don’t see what I can do on YunoHost’s package for Radarr.