Add button to non client area of another application

I recently paid attention to Team Viewer, the remote control software, and I saw that it adds a custom button to the non client area of lots of opened applications.

I have been searching for a while but I have no clue how to start, however I found this that could be an approach, but this only works for handled windows forms

http://www.thecodeking.co.uk/2007/09...l#.U0HvYvl5Mow

Thanks in advance for your help, I appreciate a lot!