Using a subdomain as the url for a second Shopify store

Hello.

I currently have a Shopify store using a custom domain I have registered through Shopify, ex. mystore.com

I want to set up a separate wholesale shopify site using a subdomain as the primary URL, ex. b2b.mystore.com

I created a DNS CNAME record “b2b” pointing to the shopify url for the wholesale store “my-store-b2b.myshopify.com

That didn’t provide a redirect and simply went to my main site mystore.com

Is there something I am doing wrong? Do I need to do anything on the wholesale store side? It won’t allow me to add the subdomain in the “Connect an Existing Domain” as the primary domain in settings saying “This domain is already connected to another Shopify store.”

Any help would be appreciated.