How to access Horizontally scale feature in font through expression?

As Rick says, you won't be able to affect that property directly, but you can use the tiny Animate mean to add a Tracking animator and then you can control the spacing that way.

If you check the Expression Language menu in the timeline, you will see that horizontal and vertical character scale are not yet available. 

Here is the Help page on controlling text with expressions.

It is a good feature request.