Just add sub nav using bp_core_new_subnav_item and modify file activity.php just put your code before if ( is_user_logged_in() && bp_is_my_profile() && ( !bp_current_action() || bp_is_current_action( ‘just-me’ ) ) )
bp_get_template_part( ‘activity/post-form’ );
with if condition like
if(is_user_logged_in() && bp_is_my_profile() && bp_is_current_action( ‘schedule_message’ )){
get_template_part( ‘custom_files/schedule_message’ );
if current action is you tab slug then load you page template otherwise default