Your question is somewhat obtuse, what exactly do you mean? There is a "To" field in the compose window and it's automatically filled in when you highlight a message and hit Reply.
I guess you mean in Zimbra?
I want change / add FIELD (header) reply-to from compose mail window. I know, I can add this value in user setup page (user account), but my users ask me about full editable reply-to field from compose mail window (like thunderbird or outloock).