Hey there Bigriggsmom,
From what we understand in the details you provided so far, you used Hide My Email to set up an account for an app, and now you want to reset the password for it.
If that's right, the way Hide My Email works is that it creates an alias email that's attached to your Apple ID email. That way, it sends any related emails to your Apple ID email, but keeps your true email hidden when signing into that third-party app you mentioned. The link above your post has more details about this: What is Hide My Email?
You'd need to use the process for changing the password for that app/service, but as you're unsure of how to do this, we recommend reaching out to that app developer to obtain the steps. Just in case, this shows how to contact an app developer for an app you downloaded from the App Store: How to contact an app developer
If we've completely misunderstood what you explained, please provide us more details. The more we have, the better we'll be able to understand the issue.
Cheers!