Link "From" and "Reply to"

I want to be able to select the "From" email address and fill the "Reply to" field as the same so it looks like the recipient should reply to the person the mail is from.

Currently my default is " myname@mydomain.com", I want to select from the drop down box " orders@mydomain.com" and automatically fill the "Reply to" box with " orders@mydomain.com".

Greg Aanes
2109 Queen Street
Bellingha WA USA