The extra fields can be injected like this:

..code-block::twig

..code-block::html+twig

{# templates/user/create.html.twig #}

{{ form_start(form) }}

{{ form_row(form.username) }}

{{ form_row(form.email) }}

{# Hidden field to send additional sponsorship code #}

<input type="hidden" name="user_create[referralCode]" value="{{ referralCode }}"/>

<button type="submit">Submit</button>

{{ form_end(form) }}