I see he has "Discord.js" as a separate file in his C3 project, and can post messages via a textbox.
ohh now i realize
I did something like this last year.
but it was the opposite.
I was sending command to construct from discord.
here example video
oyunkulturu.com/temp/discord_k.webm
yes you need to make bot with discord.js.
You can run the bot on the node platform and then do whatever you want.
If you want you can look at the code of the bot I made.
github.com/erenertugrul/construct-discord-bot
but I think it would be better if 
DrMeepso shared a sample project