SET PROMPT-RCPT-ALWAYS NO/yes  -  whether to prompt for CC and BCC always

With the usual NO setting, MM does not prompt for a CC and BCC address if
  you type an address on the same line with SEND at the MM> prompt.

If you change it to YES, MM will always prompt for a CC and BCC, provided
  the variables PROMPT-FOR-CC and PROMPT-FOR-BCC are set to YES.

Example: Set to NO:    MM>send abc
                        Subject:

         Set to YES:   MM>send abc
                        cc: def
                        Subject

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