Im a first time DM commiting the sin of running a homebrew campaign. I have included a lot of mythological creatures from various cultures and I want to include (what i think is) a skin walker from the Appalacia region in the U.S. Specifically because when you whistle at night the skin walker ( again not sure if that is the correct creature/cryptid) comes after that person.

What I want to know is how do i get them to whistle? Like those mentalist tricks or magic tricks etc. Its not a crucial part of the campaign but a super fun encounter i think. Obviously in the folklore the creature kills whatever whistles and the players in the case will be able to fight and kill the creature after a hard fight.

Can anyone help with how to do this? Thanks!

Tldr: want to know how to passively convince my player(s) to whistle for a fun encounter.

Night birds.

Night birds with pretty tunes.

They'll want to mimic, then the mimics will want them.

They meet a merchant and either he is sketchy or angry because the party bargains to much.

He tells the party to whistle at night and something great will happen. He might reason that it is local folklore etc....

Whistling is the solution to a riddle perhaps?

Buy a cheap bag of slide whistles.
Put them on the table.
..?
Shenanigans.

Have an NPC say, “I’m going to patrol over there. Whistle in a few minutes so I can find my way back.”

A riddle, where the answer to the riddle is to whistle, or is a bird call.

Ooh, night birds with a call-and-response pattern that seem to be leading them somewhere, but then the responses stop.

Progress following the call stalls, curious players want to see what happens next and try to coax the next bird along the path to call by mimicking the missing response.

You meet an old man out in the woods who has a very intelligent dog. The dog can navigate through the woods and is commanded by whistles. He invites you into his cabin and stay for the night. That night he dies in his sleep and you are entrusted to care for the dog. Now you have a reason to whistle.

More subtle. Whistle to call birds, whistle a horse, whistle mountain where whistling id forbidden.

Making things vaguely forbidden always makes players do it

Not the biggest fan witht the setup of tricking the players into such an action. Instead I would give in game leads, that whistling is a key information.

Guardsman report, after his colleague dissapeard during a watch:"I heard him whistling around the corner, then he was gone"

Surely an NPC who's not a moron will know the legend and why that's a bad idea. Unless the idea is that they hate this ignorant person.

As a contingency, if your players don’t whistle when the DM expects them to, the skinwalker could just pick off the songbirds one by one before coming after the party just because

NPC 1 sings, “Always look on the bright side of life…”
NPC 2, looking at NPC Dummy, “Dummy, no!”

Gift one of them a bird whistle or required it to solve a puzzle