How to connect only a subdomain of another domain to my Squarespace website

Hi everybody. I need to give my website a one more domain address. It has to be a subdomain of another domain. But when I add a required CNAME record to DNS (with a 947c*********8rjw string) Godaddy warns that this can affect the root domain (and probably other subdomains). Is it really so? If yes, how to modify that DNS record then?=

Thank you so much!

Regards,

Dan