I think you have that option.
Either you can provide the email address of your dentist and say that he/she will submit electronically, and they will get instructions on how to do so.
Otherwise, the old school way (which I did), they give you a LOR Matching form (kinda like the transcript) and then you take that into the dentist, and have them attach the form to the letter and they will mail it in!
There will be that option when you get to the LOR section and input the recs' infos