@dudo the details for that additionalSignUpFields Lock variable to modify the Classic Universal Login template is here: Lock Configuration Options
(Note that this is only works for Database connections)
In the UI, you would go to your tenant dashboard > Branding > Universal Login > Login (tab) > Ensure you have “Customize Login Page” enabled > Modify HTML
If you are using the New Universal Login template, however, you will not be able to add additional signup fields to the form.