add_rh_fields_billing.php

<?php

function my_pmprorh_init()
{
	//don't break if Register Helper is not loaded
	if(!function_exists( 'pmprorh_add_registration_field' )) {
		return false;
	}
	
	//define the fields
	$fields = array();
	$fields[] = new PMProRH_Field(
        "chapter",             // input name, will also be used as meta key
        "text",                    // type of field
        array(
			"label" => "Chapter",
            "size" => 20,        // size attribute of text field
			'profile'	=> true,			// show in user profile
			'required'	=> true,			// make this field required
			'memberslistcsv' => true,
        )
    );

	
	//add the fields into a new checkout_boxes are of the checkout page
	foreach($fields as $field)
		pmprorh_add_registration_field(
			'after_billing_fields',				// location on checkout page
			$field						// PMProRH_Field object
		);
	//that's it. see the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmprorh_init' );