Add hidden PMPro registration field that shows a message when a checkbox is ticked.

date-check.js

jQuery(document).ready(function($) {
	var label = $('label[for=hidden_check]');
	$(label).hide();
	$('#date_check').click(function() {
		if($('#date_check').is(':checked')) {
			$(label).css('color','salmon').show();
			$('input[name=hidden_check]').val('1');
		} else {
			$(label).hide();
			$('input[name=hidden_check]').val('0');
		}
	});
});

pmpro-register-helper-hidden-field.php

<?php // Do not include in your customizations plugin
/**
 * Add hidden PMPro registration field that shows a message when a checkbox is ticked.
 * Requires that you also enqueue the attached JS file from a folder in the plugin that you add this recipe.
 */

function register_helper_hidden_field() {
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}
	// define the fields
	$fields = array();

	// HTML
	$fields[] = new PMProRH_Field(
		'hidden_check',
		'hidden',
		array(
			'id'   => 'hidden_check',
			'label'   => 'WooHoo, Confirmed!',
			'profile'   => true,
		)
	);

	$fields[] = new PMProRH_Field(
		'date_of_birth',
		'date',
		array(
			'label'     => 'Date Of Birth',
			'size'      => 40,
			'profile'   => true,
		)
	);

	// CHECKBOX
	$fields[] = new PMProRH_Field(
		'date_check',
		'checkbox',
		array(
			'text' => '<span id="change-result">Yes, I have confirmed my DOB</span>',
			'id'   => 'date_check',
			'label'   => 'Date confirmation',
			'profile'   => true,
			'required'   => true,
		)
	);

	foreach ( $fields as $field ) {
		pmprorh_add_registration_field(
			'checkout_boxes',
			$field
		);
	}

}
add_action( 'init', 'register_helper_hidden_field' );

/**
 * enqueue_check_date_change Tell WordPress to load a javascript snippet named date-check.js that is saved in the js folder for the plugin where this file is saved.
 *
 * @return null
 */
function enqueue_check_date_change() {
	wp_register_script(
		'check-date-field', plugins_url( 'js/date-check.js', __FILE__ ), array( 'jquery' ), time(), true
	);
	wp_enqueue_script( 'check-date-field' );
}
add_action( 'admin_enqueue_scripts', 'enqueue_check_date_change' );