This is how I would do it:
Add drag and drop behavior to the scrollbar, in it's properties select Vertical only under Axes.
Now set its limits (So it can't be dragged off the screen) likes this:
ScrollBar | Compare Y > Lower Scrolling Limit --> Set ScrollBar Y to Lower Scrolling Limit
ScrollBar | Compare Y < Upper Scrolling Limit --> Set ScrollBar Y to Upper Scrolling Limit
You can then read the Scrollbars' Y coord and use it for whatever you want.
Here is a .CAPX example: dropbox.com/s/5797oxx04wmk5z4/ScrollBar.capx?dl=0
(Can't post clickable links yet)
Sorry for bumping an old thread, but I thought it might be usefull to some.