This is actually not very obvious, and that's our fault. We'll make this better in the future though. For now, here's how you do it:
First, when you setup the preorder for the album, you don't attach the full album, you attach the single you want to give away instantly. You also DON'T setup Digital Street Date. This is very important. Then the day before the digital release of the album is set to be available, go into the product, remove the instant grat single, and attach the full album. Then you setup Digital Street Date for the next day. When the next day arrives, anyone who bought the product previously, will get an email saying the album is available.