Signup with google-oauth2 while passing user_metadata variables

I am using auth0.js web library

I have implemented passing promo codes through user_metadata for sign up with email without any issue.

However I cannot figure out how to implement it for connection: 'google-oauth2'

Here is my webAuth athorize block:

webAuth.authorize({
  connection: 'google-oauth2',
  user_metadata: {
    promo_code: promoCode
  },
  redirectUri: redirectUri,
},

However I cannot seem to get it to work using google-oauth2
If I try and use webAuth.signup instead of webAuth.authorize it says I need to pass an email and password.

When I use this code, the user is able to sign up no problem however the user_metadata is not storing the promo code the user signed up with therefore being charged full price.