Custom Validation-Action Hook

Hi!

I want to add a Referral field for users to signup and enter the Username or Email of existing users which referred them to our site. I selected the Username/E-Mail field from the UM Forms.

I´m stuck on generating the custom action for this field, I just need it to check for existing usernames and/or emails and if there´s a match allow to go forward, and if not, display an error like “The user/email doesn´t exist, please double check and try again.”.

If there´s an easier way to achieve this, happy to listen to new alternatives!

Thank you very much everyone!