Is it possible to backup a disk on a Windows shared folder?

Actually I have done this with a TimeMachine, to a shared NTFS formatted drive. It's been a few years but this should still work. EG pardon the 100 foot view, I don't remember all the details.

Share the drive (or a folder on the drive, which I did) on Windows and give a username you can share with the Mac backup app. Give that user CHANGE permission on the share.

Then you create a sparse image on the Mac, mount the image once created and format it appropriately (at the time it was HFS+ only) on the Mac and then copy the DMG you just created and formatted to the share on the Windows PC/Server.

macOS can't write to a directly attached NTFS drive natively without 3rd party software, but it can write to an SMB share, regardless of the filesystem.

The DMG is required so you don't lose the APFS/HFS+ extended attributes that NTFS does'nt really support. And if you are using TimeMachine it HAS to back up to HFS+, or for more recent macOS versions APFS.

So in reality you are not backing up directly to NTFS but a Mac filesystem on an NTFS share.