If you are seeing the following screen when using the app proxy URL, you will need to make one quick configuration change.
Sometimes the settings found in the customise section of the app will not be correct for the app proxy. If the URL below does not work please do the following.
Go to apps screen
Select view details
Ensure that the field is set to returns and matches what is set in customise section of the app
Your app link will now works as expected Example: https://greatcollection.myshopify.com/a/returns
Any issues please don't hesitate to contact.