Adding stickers to your event is a feature exclusive to the virtual booth.
Step 1: Enable Stickers
You can do this by going to the "Virtual Booth" section on your event, and selecting either "Beginning" or "After Capture".
Step 2: Uploading Stickers
In order to add stickers to your event, just click the "Actions" button in the top right corner and select "Manage Virtual Assets" from the drop-down. Then select "Sticker" from the tab and click "Upload".
Tip: To manage and delete stickers you can do this from the management area as well - Event dashboard > Action (drop-down) > Manage Virtual Assets > Sticker.