To promote your course, you can use its special promo page. The link to it is https://stepik.org/course/<courseID>/promo, where <courseID> is the ID of your course.
The page is generated automatically according to the description of the course. You can access the promo page from your page by clicking Content ⇒ Information ⇒ Preview:
Promo page is available via link and is visible to learners that have not enrolled yet. As learners click the Join the course button, they immediately go to the first lesson of the course, and the promo page becomes unavailable.
An example of a promo page: