On desktop (MacOS and Windows) or on Wire for Web:
To add emojis on MacOS, go to the menu bar and click Edit > Emojis & Symbols.
To add emojis on Windows 8 and Windows 10, right-click on the taskbar and go to "Toolbars" > "Touch Keyboard" and a new keyboard icon on your taskbar will appear. Click on it to access the keyboard and click on the smiley-face icon.
Windows 7 supports emojis, but doesn't have an emoji keyboard.
For instructions on how to do this on mobile, click here.