"Use a Domain I Own" is not an option on my domain tab

I have a domain I already paid for through a third party. I would like to just link this domain to my site instead of transferring it to squarespace. All the instructions I have found to do this say to "open my domains panel" and then click "use a domain I own", however the only options I see when I go in my domains panel is "transfer a domain" and "get a domain". I even went into transfer my domain briefly to see if use a domain I own was in there but I didn't see anything in my quick glance.

Anybody have any insight into what I'm missing?