There are two ways to do that whistle and they both involve having your tongue nearly vertical in your mouth and shooting the air through a small hole between the tip of your tongue and hard palate down right past your bottom lip. The hard thing is that you have to push a lot of air for it to work like that, which is why some people use their fingers to push up and hold their tongue. Ive been whistling my whole life and only learned that one in my 20s