Browse to the board where you wish to post and click the New Topic button (positioned by default at both the top and the bottom of the board, which should take you to the Start new topic screen.

There are the 'Subject' field and 'main Message' field

Enter your subject and start to type (or paste) your message into the main text area, noting that a message missing either a subject or a body will not be accepted for posting until it has both.

If you're happy with your plain text message, you can then post it by clicking the 'submit' button.