Default permissions for newly created dirs on mounted file system

In my user account I've mounted an external file system via fstab. Now I've service running that should be able to write data on this file system. The service is running under a different user, but I've put it in a group that has write access to the mounted file system. But here is the problem: When the service creates new directories they are created under my username and with default permissions drwxr-xr-x - so the service itself can't write into the directories it just created. How do I set this up properly?

My fstab:

videos /home/user/videos 9p 9p,trans=virtio,cache=mmap 0 0