Click o the right of the Message field. To get a closer look at a sticker, click on it and do not release the mouse button.
To send a sticker, click on it once.
If you've been sent a sticker from a sticker pack that you don't have, you can add the sticker pack to yourself.
Click on a sticker and click Add.
Click "Create your own sticker pack".
Follow the bot's instructions.
Click Import Sticker Pack.