Validation of multiple field values together

[delzhand]

I would like to modify the user registration form to add a field called "claim", and I need to validate the registration form in such a way that both the username value and claim value are passed into a function I've already written. I haven't been able to find any existing instances of validation that involve multiple fields.

Is there any sort of post-submit validation hook I can use with all the submitted values available?
Validation of multiple field values together

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