Of course you can add the Hetzner Storage Box via GUI and don't need any fstab workaround. The only thing you need to remember is that if you not use the Storage Box Admin Account but a Sub-Account the share is the username of the sub account
not the folder (
https://docs.hetzner.com/robot/storage-box/access/access-samba-cifs)
Steps
1. Create a sub account in the Hetzner Storage Box Settings pointing to the folder you want to use; enable Samba. Example User: u301XXX-sub1
2. In the Proxmox GUI, create a new Storage with the following config:
ID: Name of your Storage
Server: u301XXX.your-storagebox.de
Username: u301XXX-sub1
Password: XXX
Share: u301XXX-sub1
With this config I added the storage successfully and also created the first backup of my VMs. Even if you enter the username as Share, the content is stored in the folder you assigned to the user in the Hetzner Storage Box Settings.
Cheers