If you mean that you have an email address forwarded to your "main" email address and you want to use the email address of the forwarding account in order to reply to the received email in your "main" account the answer is you can't. It has to show the email address it was sent from.
What you can do , assuming that you are using a web browser to access your email, you would need to add your "forwarding" email account to your "main" email account so that you can move between the two accounts while just being logged into your "main" email account. You would then use a drop down box in the address bar to select which account you are sending from. This in effect would mean that when your are logged into your "main" account your web browser is acting as an email client.
If you are using a company to forward your email without it being on their servers, ie just bouncing it, then as far as I am aware you will not be able to send from that address no matter what you do.
Can you explain how you did that other than if it was a true email account that could send emails independently?
That would only be possible if you had done as I suggested in my first message with a true email address and not a bounce forwarding email address.
I understand what you have said but I do not know of any way you can change a non or bounce email address into a sending address.
If you find a way please post back how it was done.