set terse-text-prompt

SET TERSE-TEXT-PROMPT NO/yes  -  whether to show the long Message prompt

With the usual NO setting, MM displays several lines of prompts at the
  point when you can type your message:

  Message (End with CTRL-D or ESC
    Use CTRL-B to insert a file, CTRL-E to enter editor, CTRL-F to...

If you change it to YES, a shorter prompt is shown.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show
(at the MM> or Read> prompts:)  send