Emojis
Your app can have its own custom emojis, which it can use in every server it's in. You can see them under Emojis in the sidebar, together with their IDs.
Kite can't upload emojis itself. Click on Create Emoji to open the emoji page of your app in the Discord Developer Portal and upload them there. New emojis can take up to a minute to show up in Kite.
Using Emojis
Buttons and select menu options in the message editor, and blocks like Create message reaction and Create poll, have an emoji picker. Your app's emojis are listed in the picker under Custom Emojis.
In message text there is no picker, so you type the emoji in Discord's format instead:
<:name:id>for an emoji, for example<:kite:1234567890123456789><a:name:id>for an animated emoji
Use the name and ID from the Emojis page. The picker only lists your app's own emojis, not the emojis of a server.