Shoutbox

Well, if that dialog is shown in the ParseCommand, ParseTag or ReceiveNotify functions of the plus api, then you just have to copy the text in the textbox into the sResult (or similar name,  don't remember) variable of those functions.

However if you need to send any text when you want, you should use the messenger api, although you can also use the sendkeys function simulate that the user has pressed the keys in the conversation window.