I don't think you can add stickers in Trello using mobile devices. You can check the answer of this question from Trello support team here : https://community.atlassian.com/t5/Trello-questions/Cannot-add-stickers-on-Trello-when-using-ipad/qaq-p/933649
Congratulations to @Laura Holton , our latest winner of Taco Tuesday! And thanks to @Kristjan Geir Mathiesen - Origo for sharing the picture of Taco having fun with his new friend...
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