<p>It’s in your best interest to email the professor as soon as possible. Say literally what you need. You’re interested in the class, but it’s full. Ask if you can add the class or if you can get some type of permission code. Basically it’s just a way to give the professor your name in case they want to add seats (sometimes they wont if class doesn’t have enough seats). They might reply telling you their procedure, but if they don’t…just go up to them on the first day and tell them you emailed them about joining the course.</p>