Execute WordPress function from URL

functions.php

<?php

// Run by doing https://domain.com/wp-admin/admin-ajax.php?action=osm_id_callback_action

// For users not logged in
add_action('wp_ajax_nopriv_osmid_callback_action', 'osm_id_callback_action');

// For users logged in
add_action('wp_ajax_osmid_callback_action', 'osm_id_callback_action');

function osm_id_callback_action()
{
    // Do something
}