Block direct access to checkout with custom subdomain

Is there a way to block direct access to a custom subdomain? We have our shopify store working as expected, when a user is ready to buy we send them to

checkout.samplesite.com/abcd/checkout/abcdefg

That works excellent, what we would like to do is prevent users from going directly to

checkout.samplesite.com

Is that possible within Shopify settings or do we need to make some DNS changes with the registar?