Alert Procedure:

For in-person games the PARTNER of the person who makes an alertable bid is the one who says "alert". You do NOT need to say anything further unless asked to do so by an opponent. At that point you should explain your partnership agreement as to what the bid shows. If you forget to alert and your side becomes the declarer you should announce to the opponents the failure to alert before the opening lead. If your side becomes the defenders do not say anything until the completion of the play of the hand. There is no automatic penalty for failing to alert. However, if there is a problem with an alert, or lack of an alert, that would have changed the result you should call the director who can adjust the score. If you forget an alertable bid you are only obligated to tell the opponents what your agreement is. You are not obligated to tell the opponents what you actually have.

For online games the person who makes the alertable bid is the one who alerts. You can input explanation in the box before you bid or use chat feature. Just be sure you chat to "opponents" and not to "table".