You can add a sticker to a Gift before sending it to a friend. Each sticker can only be used once and will disappear from your inventory when you send a Gift with it. You can only hold 25 of the same Gift sticker type at a time.
Trainers can obtain some stickers from opening Gifts from Friends, while other stickers can only be purchased in the shop.
Note about child accounts: Trainers on child accounts are only able to attach stickers to Gifts if their parents or guardians opt into enabling social features through the Niantic Kids Parent Portal or Pokémon Trainer Club website.