bp_get_the_message_thread_mark_unread_url( int $user_id = null )

	function bp_get_the_message_thread_mark_unread_url( $user_id = null ) {

		// Get the message ID.
		$id = bp_get_message_thread_id();

		// Get the args to add to the URL.
		$args = array(
			'action'     => 'unread',
			'message_id' => $id
		);

		if ( null === $user_id ) {
			$user_id = bp_loggedin_user_id();
		}

		$domain = bp_core_get_user_domain( $user_id );

		// Base unread URL.
		$url = trailingslashit( $domain . bp_get_messages_slug() . '/' . bp_current_action() . '/unread' );

		// Add the args to the URL.
		$url = add_query_arg( $args, $url );

		// Add the nonce.
		$url = wp_nonce_url( $url, 'bp_message_thread_mark_unread_' . $id );

		/**
		 * Filters the URL used for marking a single message thread as unread.
		 *
		 * @since BuddyPress 2.2.0
		 * @since BuddyPress 2.9.0 Added `$user_id` parameter.
		 *
		 * @param string $url     URL used for marking a single message thread as unread.
		 * @param int    $user_id ID of the user relative to whom the link should be generated.
		 */
		return apply_filters( 'bp_get_the_message_thread_mark_unread_url', $url, $user_id );
	}