To ask for a sticker you can act in 2 ways:
1) ask your friends via normal message (a friend can send you a sticker every 24 hours).
2) Or click on the sticker you are missing, the following window will open on the whole screen
below will appear the orange button ASK THE CLUB, by clicking on it your request will appear in the new Chat which is next to the chat Club (as shown in the following photo)
You can ask the CLUB for a card every 8 hours.