What kind did you get?
The type I have use a flexible heating element that just wraps around and gets glued on. On the left side it goes directly on the handlebar under the grip. On the throttle side it goes on the outside of the throttle tube, under the grip. I've never seen a style that was meant to go under the throttle tube. I would think that anything between the throttle tube and the handlebar would interfere with the ability of the throttle tube to move smoothly.
If you have the tube type that are molded to fit 7/8" handlebars then i don't think you'll be able to make it work for the throttle side. That type is meant for snowmobiles with a thumb throttle that don't have the larger diameter throttle tube on top of the handlebar.