Select email to send Mailbox invite from

tl;dr My private email was almost exposed when trying to send an invite link to Mailbox. Can there be a way to select which email address is used to deliver it to others? Can a one-time code be generated?

I've been using Mailbox for a year and some change, and have started getting people to consider ditching Gmail for it little by little. During walking someone through the signup today, they asked if I had a voucher code. I had never used it (didn't know about it), but of course I wanted to give my friend a deal! I was concerned about sending it to them, because my primary email is used only for logging in to here.

If the use of the default email address is required to limit and keep track of voucher activations, then is there another way to copy and share the one-time code?