Effect

The magician sticks the coin into the hanky, turns it over and then the coin disappears from the hanky.

Secret

You secretly rap a rubber band around your fingers and thumb.

Then you place the hanky in that hand. With your other hand place the coin into the handkerchief.

Then let the rubber band slide off of your fingers and onto the hanky, so it surrounds the coin and it is under the hanky.

You then slide your hand up to the end of the hanky and shake it. This makes it look like the coin has disappeared. But it is really stuck in the handkerchief, held by the rubber band.