Send user ID to Stripe as metadata.

my_pmpro_stripe_params_send_user_id.php

<?php

// Copy from below here...

/**
 * Send user ID to Stripe as metadata.
 * 
 * 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_params_send_user_id( $params, $order = null ) {
	global $my_pmpro_checkout_order;
	if ( empty( $params['metadata'] ) ) {
		$params['metadata'] = array();
	}

	if ( empty ( $order ) ) {
		// Save order while creating payment intent.
		$order = $my_pmpro_checkout_order;
	} else {
		// Use saved order while creating subscription.
		$my_pmpro_checkout_order = $order;
	}

	$params['metadata']['wp_user_id'] = $order->user_id;
	return $params;
}
add_filter( 'pmpro_stripe_payment_intent_params', 'my_pmpro_stripe_params_send_user_id', 10, 2 );
add_filter( 'pmpro_stripe_create_subscription_array', 'my_pmpro_stripe_params_send_user_id', 10, 1 );