Add a "One-TIme to"

I need to add a “One-time To” line . One-time BCC is optional. We may keep it or remove it. Below mentioned is the screenshot.

The user want to communicate with the user by sending mail them as TO not in CC.They are communicating the user using “One-Time cc now”.So it is sending mail in CC to user. Please let me know how can I add it.![