If you already teach in-person courses, send an email (or even make phone calls!) to all your existing and past students inviting them to take your courses online. You can offer a discount for early sign-ups or for students who take your first beta course.
If this is your first course ever, send individual invitations to handpicked students (friends and family and any collectors you may already have) and invite them to join you for your first online class. An announcement across your social media platforms is another way to garner interest.
You can also try piquing interest by recording a short promotional video to share on YouTube, Facebook, and Instagram (or any other social platforms you already use frequently) with a teaser of your course.