No worries all, I probably really confused it.
I just added a Text Field and added a CSS class of hidden-field and then just did this:
add_filter('acf/update_value/key=XXXXXXXXX', 'my_acf_update_value', 10, 4);
function my_acf_update_value( $value, $post_id, $field ) {
if( !$value ) {
return uniqid();
} else {
return $value;
}
}