We are looking to add a custom validation step to the registration process. The requirements are:
1. Add a new field to the registration screen to capture a numeric value (external ID). This sounds like it is trivial using Liquid.
2. The value they entered for external ID needs to be validated to a database we store outside of Shopify
3. If the external ID doesn't exist, the user can't register. If the external ID does exist, we actually need to return a value to shopify that needs to be stored in the user profile.
What is the best way of enabling this type of functionality?