How To Use a Hidden Email Address In a Return Message?

Used hide my email for the first time the other day to make an online purchase. The item needs to be returned and I just now received an email from the merchant regarding same.
I hit the “return” button and my real email attaches. I use the data arrows to see if I can select the HME choice and it does not appear. I’ve tried to erase those email suggestions and place the HME given to me without any success.
How do I maintain my hidden email address in replying to an email?
What’s the purpose of a “hidden email address” if it only works once? So thinking there is a way!
—–
Greg