custom validation field

Hi,

I’m trying to add a custom validation field in the registration form of my website. I want to add a “Invited by” field in order to have control of people that it’s invited by someone. What I have to do? Could you help me with an example, please?

I just want that if the new user put a user name into the field, it must to be checked before the user registration, the user name must exist, in order to avoid possible mistakes.

I know that I have to change the functions.php of my child theme, but I don’t know exactly the way to do that…

Thanks in advance.

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