You must add the domain to your account to be able to create the subdomain and host a website for this. Fortunately, you can do so without affecting your current website.
You can simply add the domain to your account using CNAME verification.
One the domain has been added to your account, you can create the subdomain on our end.
As long as you don’t modify your existing DNS records (which you don’t need to do), there is zero impact to any active services on your domain name.