how do I log into my iPad app after I created the log on using hide my ID from a different device?

When initially logging-in to a website, your Apple devices will automatically save the log-in credentials to the Passwords App (that integrates with your Apple Keychain where the credentials are actually securely stored). When using Hide My Email to create the login, the Hide My Email address used for the username is captured.

However, if you use a Hide My Email address for an App Username, you must be careful to ensure that the Password that you create is saved in your Passwords App - saving the credentials manually if necessary.

Credentials are captured automatically for the majority of websites. Apps by contrast might not be signing-in using a webpage - and it is these Apps that you must manually save the associated login credentials.

If you can't find the App login credentials in the Passwords App, you have no alternative other than to follow the "lost password" procedure that may be unique to the App. Perhaps the App has an associated web-page? If needed, seek assistance from the App Developer or the App's own support resources:

How to contact an app developer - Apple Support