Validating a field entry with data from a separate Google Sheet

We have 2 tables:
Registered attendees with unique identifier codes
Registered products for testing with unique identifier codes

We are looking to build an online check-in form that:

  1. We scan the attendee’s unique identifier into the first field and the exit of that field would activate a validation lookup from the attendee Google Sheet and either enter a checkbox to confirm that this code is in the system or not before heading to the next field
  2. Following the validation (or not) of the Attendee, we enter the Product field and scan in the product identifier into the next field and like the attendee field, when exiting that field a validation process will lookup the code entered in the Product Google Sheet and either enter a checkbox that this code is in the system or not before the entire form is submitted

Is it possible to incorporate real-time validation into an online form where data is being queried in 2 different Sheets before the completed form is actually submitted?

All of that is possible.

OK, where do we learn how to build this form in AppSheet?