Shoutbox

If using Messenger Plus API (ReceiveNotify):
Fill the sTextToSend buffer.

If using Messenger API (OnIMReceived IIRC):
When you handle the receipt of a message, record which e-mail address this is from. Then, when you wish to send the reply, use the API to initiate a conversation with this person (if one exists, it will just find that). The focus will now be in the right place to use SendKeys or a similar method of 'typing' the text you wish to send.

Hope this helps.