How to call wordpress function directly from url?

I have added custom payment method to woocommerce and its working fine . I just have one problem that it calls a callback url for saving transaction information to db. I have created new function for this in my plugin file but i cant excess it directly .
This is how i have done it:

//add_action('wp_ajax_nopriv_payment_callback_action', 'payment_callback_action');

//function
function payment_callback_action() {
echo "Its Working!";
}

I am trying to access it by :
url:"<?=site_url( '/' );?>wp-admin/admin-ajax.php?action=payment_callback_action

It seemd that it because of i dnt have privillage to use it directly but how can i do this ?.
Thanks