new pledges

Hi,
I am trying to connect a pledge post to the registered user how donate it.
It will look something like that:

function m_add_user_to_pledge() {
global $post;
$post_id = $post->ID;
if(!get_post($post_id) || get_post_type($post_id) != 'donation')
return false;
$current_user = wp_get_current_user();
$user_id = $current_user->ID;
update_post_meta($post_id, 'donor', $user_id);
}
add_action($hook, 'm_add_user_to_pledge');

What $hook value do i need to use (the name of the function that creates a new pledge)?

Thanks