Is there any way to get the thumbnail url from uploaded images in post to automaticly be included in the value? If there is no image uploaded in the post then the value an be empty

add_action(‘publish_page’, ‘add_custom_field_automatically’);
add_action(‘publish_post’, ‘add_custom_field_automatically’);
function add_custom_field_automatically($post_id) {
global $wpdb;
if(!wp_is_post_revision($post_id)) {
add_post_meta($post_id, ‘field-name’, ‘custom value’, true);
}
}

Hope that helps others as well

I tried:
add_action(‘edit_post_link’, ‘mk_set_default_custom_fields’);
without success.

Reference: edit_post_link at http://codex.wordpress.org/Function_Reference/edit_post_link