Combine ground beef, bread crumbs, onion, milk, egg, 1 tablespoon dried Greek seasoning and salt in a large bowl, mixing lightly but thoroughly.
Pour 3/4 cup water into the Instant Pot; set rack in water.
To make a foil sling, cut a 28-inch piece of heavy-duty aluminum foil (or layer two pieces of regular foil) and punch 6 to 8 holes in it using a kitchen fork or small knife.
Form beef mixture into an 8 x 4-inch loaf on the foil sling over the holes. Lower the meatloaf with the sling onto the pressure cooker rack. Close and lock pressure cooker lid.
Use beef, stew or high pressure setting; program for 25 minutes. When time is up, let the meatloaf stand in pressure cooker for 10 minutes.
Meanwhile, combine yogurt, cucumber and remaining 1-1/2 teaspoons Greek seasoning in medium bowl. Season with salt, as desired. Set aside.
Use quick-release feature to release pressure on the Instant Pot; carefully remove lid. Using foil sling, transfer meatloaf to cutting board.
Remove foil and cut meatloaf into 8 slices. Serve with Cucumber-Yogurt Sauce. Enjoy!