Shoutbox

Click the Messenger Plus! Live icon in your contact list, then click Preferences & Options

Click the 'Conversations' tab along the top and then 'Quick Texts' on the left.

You can then add texts and define the shortcut for them.

For example, if you tell people your mobile number a lot, but can't remember it, you could set it as a quick text with the shortcut (mobile) and then whenever you type (mobile) in a conversation it will replace it with your mobile number.