Pass extra metadata to signup

I’m trying to pass an extra “signup guid” to auth0 login/signup that I want to retrieve in a rule.
This works fine with logins:

            signupGuid: signupGuid,

lets me retrieve it in a rule using
let signupGuid = context.request.query.signup_guid;

However, if I set the screen_hint: "signup" and auto-create/sign in a new user using a database connection, I don’t get the context.request.query.signup_guid anymore.
So my question is: Is there an alternative way for me to send aribtrary data to the signup process that will be preserved by auth0 and accessible in a rule?

I think this is the same topic: Pass custom options to UniversalLogin (filled-in referal field)

However, I don’t understand to set

      authParamsMap: {'referrerId': 'abc123'}

A bit more info:
Adding a log statement to the universal login html:
console.log(config.internalOptions); shows me that the data is there, but it’s somehow truncated during the signup process?