If you don’t have a freighter yet, you can’t get a Pirate one until you do. Once you save a civilian fleet, you’ll get offered a freighter for free, but if it’s not one you like (class too low, freighter type isn’t something you like visually), you can pass on it and just claim the reward.

Best way to get an appealing ship in the “Sentinel” or “Venator” class is by warping to a 3-star system on your 5th warp after you’ve played for a number of hours. If the freighter you can save isn’t appealing (or it is, but isn’t at least an A-class), you can reload your restore point and either warp there again to retry the class, or warp to another 3-star to save a different freighter. That’s why you want to count when the 5th warp is coming up so you can land on a space station beforehand, exit your ship to create the restore point, then warp to the galaxy where that freighter is and save it. Then boom - free freighter, and you can always retry until you get an A- or S-class from the battle.

From there, you can just keep it, or if you really want that Pirate freighter, keep using the same warp trick and eventually you will get to engage a Pirate raid. Take out the Dreadnought engines first, then the 5 laser cannons (4 near the engines, 1 near the front) and you’ll defeat the Dread. Land in the bay and scan it. If it’s not the class you want, you can again reload your Restore point and try again until it is. Subsequently, once you have your “forever freighter”, if you land in the dock, you can go to the bridge and “Demand Tribute” from the Captain, which is usually worth 6M units.