Allowing a third party to own the Cart Page - custom coding to link to new URL?

Site URL: https://www.grindstonewines.com/cart

I have a couple of my pages on my site linked to our POS system (for sales & inventory purposes) but lastly I need out checkout Cart page linked to our POS system. I have the new URL but Square Space does not offer to link the Cart Page like they do with other pages. Would custom coding help? Has anyone else come across this issue? Thanks in advance