This isn’t in the GUI (yet?), so don’t open it up and expect magic mushrooms and smiley-face pills to pour out at you on a rainbow. If you want to use Direct Seeding, you’ll have to script things. But it’s pretty easy! If I can do it, anyone can.
I’m not going to go through setting up a Domain Controller or Clustering or installing SQL here. I assume you’re already lonely enough to know how to do all that.
The script itself is simple, though. I’m going to create my Availability Group for my three lovingly named test databases, and add a listener. The important part to notice is SEEDING_MODE = AUTOMATIC. This will create an Availability Group called SQLAG01, with one synchronous, and one asynchronous Replica.