set use-editor-always

SET USE-EDITOR-ALWAYS NO/yes  -  whether to use the editor all the time

With the usual NO setting, MM puts you in "text mode" to write your
  messages.  You can use the editor by typing control-e, or by using the
  EDIT command at the Send> prompt.

If you change it to YES, MM will start the editor automatically, and you 
  never enter text mode.

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