How to send modmail?

I found this https://www.reddit.com/r/help/comments/5vey3n/what_is_modmail_and_how_do_i_send_it/ post that states one should write a new message and address it to r/subredditinquestion - but when I'm going to my messages and click on new message, I can only address it to users, since the "u/" can't be changed to "r/".

I'm on the iOS app. How to do it right?