af/form/hidden_fields

Triggered right before the fields and after the description.

<?php

function hidden_field( $form, $args ) {
    echo '<input type="hidden" name="some_hidden_field" value="with some value">';
}
add_action( 'af/form/hidden_fields', 'hidden_field' );
add_action( 'af/form/hidden_fields/id=FORM_ID', 'hidden_field' );
add_action( 'af/form/hidden_fields/key=FORM_KEY', 'hidden_field' );