This recipe adds a custom to the [pmpro_member] shortcode.

custom-pmpro-member-shortcode-field.php

<?php
/**
 * This recipe adds a custom to the [pmpro_member] shortcode.
 *
 * 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_member_shortcode_custom_field_1611286235606( $r, $user_id, $field ) {
	if ( 'user_id' === $field || 'ID' === $field || 'member_id' === $field ) {
		$r = $user_id;
	}
	return $r;
}
add_filter( 'pmpro_member_shortcode_field', 'my_pmpro_member_shortcode_custom_field_1611286235606', 10, 3 );