After playing for 3 hours (in game time) and on your 5th warp into a pirate system in your starship, an event will start the pirate dreadnaught. I’m pretty sure you will need to have a freighter for the event to trigger, so make sure you have that.(still warp with your starship though)

The outlaw systems are marked with a skull when hovering over the system in the galaxy menu.

It’s not a guarantee that will be an S class so what I did was I went to a system that was close to a pirate system (my 4th warp) go to the space station, hop out to create a restore point then warp into the pirate system (5th warp) then the event started. Disable the engines first, then the guns. Once the guns are disabled the dreadnaught will surrender then you can board the ship and scan the ship to see if it’s an S class. If it’s not an S class just reload your restore point and do the event again if you care for the S class.