Enhance a C++ windows app - handle click on element which is not yet clickable

I have a source code of Win32 application (winapi). It runs a color picker. See attached screenshot. When user clicks the right-most bar, he can drag the mouse up and down to select different tone of the given color. The triangle updates accordingly. I need to add a feature that even the triangle area can be clicked with the very same behavior. So user could not only click the thick color bar, but also the area where the triangle appears, and the color tone will be adjusted accordingly. I have highlighted the triangle by a red circle so you can see what I'm talking about. I will provide full source code to the selected programmer.