This feature is available on Plus plan ($10/mon) onward. Customers are prevented from creating an account from their side and you can help them to do so to manage registrations more effectively.
Go to the app and scroll down to the bottom of the left panel. Select SETTINGS => General Config.
Set the function to Yes and the Create an Account will be hidden under the Login form:
And when visitors go directly to the Registration Page /account/register they will see the message.