Can I use a subdomain like shop.mybrand.com for my store?

Only once the root domain (mybrand.com) is already connected through WCart. The hint under New domain on Settings → Custom Domain says the same.

• Root domain already connected: enter shop.mybrand.com and click Change domain. It's added inside the existing setup straight away, with no nameserver change.
• Root domain not connected yet: a subdomain on its own can't be added. You'll see a message asking you to connect mybrand.com first, or to contact support to connect the root domain so subdomains can be added.

If this is your first domain, use the root domain. Type it without https:// (for example mybrand.com). The Custom Domain plugin must be active.