Hi @Darren_George ,
I’m guessing you want to add a “network share” so you can play music files with Roon right from your DS718+?
If so, you don’t need to refer to the specific volume on the Synology. Just use the IP-address or the (host)name you have given your Synology. Here are two examples which may help:
\\192.168.168.54\Music Backup\
The number is just an example and different in your network. The use of the IP-address number will only work if the Synology will keep getting this same IP-address every time on reboot of your router or Synology. It depends on how your network is setup, static IP-address in the Synology for example (use this only without conflicting any other devices their IP-addresses in your network). Or give it a static IP-address in your router using something like IP reservation in your DHCP settings of the router.
Easier is this way:
\\DiskStation\Music Backup\
If “DiskStation” is the name of the Synology DS718+, by default they always where during setup. This way your router and Synology can reboot also, knowing where to search for the files in your network. Just don’t give other devices that same hostname your Synology has got in your network 
And just make sure you entered the correct credentials if you have set this up for that main Music Backup folder. If it has got so called "guest’ access, you won’t need to fill in a username + password.
After this you will be able to select a subfolder in your Music Backup folder, in your case the folder “backup”.
Hope this helps!