To change your custom ‘From:’ address when replying or forwarding, follow these steps:
Log in to Gmail at http://mail.google.com.
Open the message.
Click Reply, Reply to all, or Forward.
Click change next to your address in the From field to select the address you’d like to use to send the message.
Compose and send!
(you may need to set up the custom alias email address first. you'll do this in your account Settings)