I guess the point is that you don't have to be musically trained. Which you don't. It helps.

You can tell the co-writer the time, and key of the song. Of course we can always change the Key according to the singer.

As a co-writer, I'm still with the attitude that I would rather have my co-writer sitting with me in the same room at the same time across from each other. That way, I can hit some chords and make a melody on the spot. If it isn't right, I'll try something else.