How does replying to message in external editor work?

How does replying to message in external editor work? I tried to follow these instructions but I'm not sure how it works. I tried editing the config file to :

REDDIO_EDITOR=nano

but I guess that's not the right way to do it.

Reply to a private message in a text editor by omitting the -t option The editor is choosen by looking at the following variables: REDDIO_EDITOR -> editor (config variable) -> VISUAL -> EDITOR reddio message t4_anid36