It’s OK I figured it out. I filtered the bp_get_messages_subject_value
Thanks
// adds a dynamic post name to subject field in message compose
add_filter('bp_get_messages_subject_value', 'add_post_name', 1 );
function add_post_name($subject) {
echo $subject = "Brian was here again, whoohoo";
}
Thanks but I should be more specific. I want to show a default subject field in a private message that the user is composing. I want to avoid changing the core compose.php and use an add_filter instead if possible