Include subscription metadata in Stripe Checkout session parameters

my_pmpro_stripe_checkout_params_send_user_data.php

<?php // copy from below
/**
 * Include metadata in Stripe Checkout session params.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_stripe_checkout_params_send_user_data( $checkout_session_params ) {
	$metadata = array();

	// Get value of my_field from $_REQUEST.
	if ( isset( $_REQUEST['my_field'] ) ) {
		$metadata['my_field'] = sanitize_text_field( $_REQUEST['my_field'] );
	}

	// Bail if there's no metadata to add.
	if ( empty( $metadata ) ) {
		return $checkout_session_params;
	}

	// Add metadata to Stripe checkout session data.
	if ( ! empty( $checkout_session_params['subscription_data'] ) ) {
		$checkout_session_params['subscription_data']['metadata'] = $metadata;
	} elseif ( ! empty( $checkout_session_params['payment_intent_data'] ) ) {
		$checkout_session_params['payment_intent_data']['metadata'] = $metadata;
	}

	return $checkout_session_params;
}
add_filter( 'pmpro_stripe_checkout_session_parameters', 'my_pmpro_stripe_checkout_params_send_user_data' );