set prompt-for-bcc

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

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

If you change it to YES, a prompt for BCC 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 prompt-for-cc
(at the Send> prompt:)  bcc
(at the MM> or Read> prompts:)  send