How do I send an email using my existing 'Hide My Email' address?

Hide My Email addresses are typically intended to only be used with a single recipient. If you want to get back to the original recipient associated with the hidden address you want to use again, you will need to reply to an email they have sent you. If you don't have that, or want to use the hidden address with a new recipient, I fear you may be out of luck. If it's the new recipient thing, you could try opening an email from the original recipient and changing the subject and the 'To' field, but I suspect you might run into problems...