Heya, a command like this should work:
!commands add !funfact $(urlfetch https://api.rtainc.co/twitch/random?format=[0]&choices=funfact a,funfact b,funfact c)
The random values are comma seperated.
There is a max length of the command field so you might need to shorten the link by using for example bit.ly.
A maybe better solution would be using the Quote system: [CustomAPI] Quote system
Instead of !quote you can name the command !funfact,you can easily add and edit them in your chat instead of having to change the url everytime.