http://5yfi2pfot2rxkvjmifsxvap3ifxn6sxqapjfcgjg3t5fr7bdlu6b4sqd.onion/app/signup/help
The Compose Window allows you to set, among other things, the message recipient's address (who you're sending mail to), the message subject, and the message contents. Fill out the appropriate fields at the top of the window (only the To: address is required, but the Subject: is suggested) and type your message in the large lower field.