This sign represents attaching a poster or a flyer to a bulletin board.
Press thumb tacks into the top two corners of a poster on a
bulletin board, then press in thumb tacks on the bottom two corners.
Do you post your phone number in the bathroom?
You can use the sign POST to mean "bulletin board" by signing POST and then drawing a big square in the air with
both index fingers starting at the top middle moving to the sides, then down,
then back to the middle.