If you want to include a digital pre-order with a music merch item (such as the vinyl or CD version of the album), start by following the instructions over here. Then, in the package editor, choose the corresponding album from the “What album is it?” dropdown. We’ll then give fans an instant download of the pre-sale tracks with their physical purchase, and email them a link to download the full album whenever you release it.
You can also set up pre-orders for physical versions of albums that are already out on digital. Just set the package’s release date to whatever future date you like, and that’s that. At the moment, it's not possible to include a digital pre-order with a non-music merch item.