If you know programming, just apply a hook onto the contact list or whatever window you added the button to, then in your callback function just look for the WM_COMMAND message, and check if the ID matches the CmdId attribute the <button> has (for example <Button CmdId=10001>). If it does, do whatever the button should do.
Also make sure that the message doesn't get confused with another message from Messenger Plus! or Messenger itself.
1337 is taken by Stuffplug 3
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