By accident (because back to SMB/CIFS didn’t work for me at all initially), I learned that it is indeed possible to specify subdirectories directly in the “Remote share” field with a forward slash
In @djripcord’s example, I am pretty sure
Server_Backups/Sub-dir1/Home_Assistant_Backups
should work.
In this case Server_Backups is the name of the actual SMB share, and Sub-dir1/Home_Assistant_Backups would be the directory tree.
It worked for me (see Backup to network location (SMB/CIFS share) now working - #3 by Palmin)
I think the documentation and the wording in the dialog could be improved, other SMB client implementations have separate fields for Share and Subdirectory, which gets rid of this confusion.