How to allow learners to select enrollment track?

Hi
I am using Koa release and had successfully created course with audit and professional modes. Now, I would like to allow learners to upgrade their enrollment track (from audit to professional) when they click button “Enroll Now” in course page. I see that feature in edx.org site but don’t find in documents how to setup that feature.

Anyone can help me please? Thank you very much in advance.
HP.