Tested on Linux Mint 19 x64 but should work for any OS with OnBoard onscreen keyboard.
To display emoji you need a font which contain emoji symbols. If you don't have one already, run command:
sudo apt-get install fonts-noto-color-emoji
Now copy all layout folder content to ~/.local/share/onboard/layouts folder.
You might then want to set a keyboard shortcut to launch Onboard itself, but that's outside the scope of this document.