set prompt-for-fcc

SET PROMPT-FOR-FCC NO/yes  -  whether to prompt for a FCC

With the usual NO setting, MM does not prompt for a FCC during the send
  routine.  (You are normally prompted for TO, CC, SUBJECT and MESSAGE.)
  You can insert an FCC field with the FCC command at the Send> prompt.

If you change it to YES, a prompt for FCC will always appear.  The field
  is optional, so you can press RETURN at the prompt to omit BCC for any
  message.

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