Currently while using Strapi Cloud you are provided a cloud app name and subdomain such as:
While Strapi Cloud does not natively support setting a custom 3rd party domain (though we do plan on adding this feature at a later date!) it is possible to currently configure a CNAME record in your existing domain's DNS settings.
Actual implementation of this CNAME record will largely depend on who your domain and DNS providers are. For more information on how to set a CNAME record, see the following article from Google here.
Please note that at this time it's not possible to build these CNAME records if you are currently using Cloudflare as due to Cloudflare's DNS rules it will prevent CNAME records pointing to other domains that already exist on their network and Strapi Cloud does currently use Cloudflare as well.