First step: Create a shared folder on the Synology NAS Then go to the settings of this folder, there you select NFS, there you add a client, for host adress you enter the IP adress of your HomeAssistant

On the bottom of the screen you see the mount adress of this shared folder (mine was /volume 1/homeassistant

Second step

With that information go to HomeAssistant, settings, storage and add a network storage Any Name, IP-Adress of your Synology, NFS, and the above mount adress of the shared folder