wasp/web/docs/auth/_user-signup-fields-explainer.md

userSignupFields defines all the extra fields that need to be set on the User during the sign-up process. For example, if you have address and phone fields on your User entity, you can set them by defining the userSignupFields like this:

import { defineUserSignupFields } from '@wasp/auth/index.js'

export const userSignupFields = defineUserSignupFields({
  address: (data) => {
    if (!data.address) {
      throw new Error('Address is required')
    }
    return data.address
  }
  phone: (data) => data.phone,
})

import { defineUserSignupFields } from '@wasp/auth/index.js'

export const userSignupFields = defineUserSignupFields({
  address: (data) => {
    if (!data.address) {
      throw new Error('Address is required')
    }
    return data.address
  }
  phone: (data) => data.phone,
})

Read more about the userSignupFields function here.