CNAME info for landing page not acceptable when editing DNS info

In my landing page directions for creating a new DNS record, the CNAME host I am supposed to enter is my full website & sub domain. I’m getting an error message when trying to add this field, as full domain info is not acceptable. I’ve tried adding just the sub-domain, and though I can enter it this way, it will not verify even after 48 hours.
Is there anything I should be doing to make this work?