How to add a custom field that contains a predefined custom code (like a key)

Hello,

How can I add a custom field in EasySocial registration where the user will only be allowed to register if they entered a custom code (like a key) that is provided prior to the registration?
The reason for this is because we do not want just any user to register on the site without invitation. Therefore, we will provide the user with a code that will be used during registration. if the code matches what we sent, then they can register, otherwise, they will not be registered since the custom code will be set to required.