This is not a solution but a work around for my problem.
I found out my project uses dialog-based windows to handle input data.
From that I can create a static control as a LABEL for my edittext control (http://msdn.microsoft.com/en-us/library/ms997560.aspx) with SS_NOTIFY style (to capture mouse click messages) right before my edittext control.
Then in my custom DialogProc function (http://msdn.microsoft.com/en-us/library/windows/desktop/ms645469(v=vs.85).aspx) I can handle what will happen if the user click/double click the LABEL of my edittext. For my case, I use EM_SETSEL message (msdn.microsoft.com/en-us/library/windows/desktop/bb761661(v=vs.85).aspx) to select everything in the edittext control
Because I cannot figure out how to capture messages sent when user clicking inside an edittext control, I have to implement it on the LABEL instead. This does not solve the root of the problem but at least it works and its not a bad work around.