can't set up CNAME within register openSRS?

can't set up CNAME within register openSRS?

Purchased domain name through squarespace, which uses openSRS as register.  When I go to load the Name/Host/Alias and the value/Points to/Destination, there is no place to do that.  The CNAME gives me the ability to add a subdomain under the CNAME area and the hostname is set up with: ext.squarespace.com.  How can I verify my domain with you so that I can utilize Zoho Mail as my service?