How do I add a horizontal scroll on the bottom where I used the configurable list widget?

There is a scroll bar at the bottom of your form. The scroll bar is added because the width of the Configurable list widget is bigger than the width of the form. 

Do you want to add a scroll bar on the bottom of Configurable list widget? Unfortunately, that is not possible. 

If I did not understand your question correctly, please explain in more details what are you trying to achieve.