Yes, you can. On the Course edit page, add your own coupon code in the field "Free Course Coupon Code". This works only for Single-session Courses.

Case: Imagine you've set a coupon code on your Single-session Course. Your coupon code must be a string of characters without spaces. You activate this Course and promote it in your social circles with a link to the Course and the coupon code. A potential customer saw your social post and decides to use the link. They click the "Book now" button on your Course and proceed to the Course booking page. There they have to click the plus icon to enter the coupon code you posted. When they finalise the booking process, as per usual, there will be no payment and their Course becomes immediately active. They can now book a Session for this Course by booking one of your Available Sessions.

Note i: The Student can buy this Course for free only once. If they bought this Course before and paid for it, they can still book this one for free using the correct coupon code.
Note ii: if you don't want to offer this Course anymore with the coupon code, simply edit your Course and remove it.