how to pre-populate a customer user attribute from a querystring for a SUSI policy.

[MicroTrendsTom]

User Case - User Referral Pattern
A customer-facing application needs to persist a "ReferredBy" attribute during UserSignUp - pre-populated or edited.
The business case is to track signups and sales for users for an affiliate identified by "ReferredBy"
Actually, achieve this server-side in asp.net core, but a SUSI + serverless API connector pattern would be so much better.

Add a custom attribute "ReferredBy" to the SUSI policy

1. user runs the SUSI via SignUpSignIn button on asp.net core
2. ASP.Net Core + MSIdentity - User clicks SignUp
3. SUSI UI displays Attribute appears on form for SignUp
4. user completes signup
   5.Attribute value is persisted

N.B: The info gets posted via APIConnector and persists and upserts the referralcode associated with email in a datastore.

Sense Check How to?
The problem is when passing in as a query string... assuming it can be done.
What's the best pattern for this?

Current implementation passes in like so:
https://account.algotradingsystems.net/MicrosoftIdentity/Account/SignIn?ReferredBy=acme

Note: have viewed some info on the web but goes into invitations and so on or using only SignUp policy
so the reason for posting is there a simple low-hanging fruit way to do it?

Some references looked at:
https://damienbod.com/2021/11/15/add-extra-claims-to-an-azure-b2c-user-flow-using-api-connectors/
https://learn.microsoft.com/en-us/azure/active-directory-b2c/configure-user-input?pivots=b2c-user-flow
https://learn.microsoft.com/en-us/entra/external-id/customers/concept-user-attributes
https://learn.microsoft.com/en-us/azure/active-directory-b2c/user-flow-custom-attributes?pivots=b2c-user-flow
https://stackoverflow.com/questions/66449383/how-to-populate-azure-active-directory-b2c-user-custom-user-attributes
https://github.com/Azure-Samples/active-directory-b2c-advanced-policies/tree/master/wingtipgamesb2c/Policies