If you have a Pro level subscription, you can verify the apex domain once then add as many subdomains as you like without needing any further verification. For example, adding example.com then enables you to add subdomain.example.com without additional verification.
If you have a Core level subscription, each subdomain is considered discrete and needs to be independently added to your dashboard and verified. For example, adding example.com does not enable you to search subdomain.example.com, you'll need to add that domain to your dashboard separately.