The following should do it
twapi::configure_window_titlebar $hwin -contexthelp 1 \
-minimizebox 0 -maximizebox 0
where hwin is the toplevel window handle. Note -minimizebox/-maximizebox
must be set to 0 (that's a Windows limitation).
However, I'm not sure the above is useful as I don't know how to get a
click on the button to invoke code in your application.
An alternative, if you are using Tk, might be to use a [place] command
to overlay a help button image appropriately in the title bar but I'm
not sure how feasible it is to figure out the exact position it would
need to be placed.
/Ashok