States the following:
To find the EVID of an App Store app, use one of these procedures:
If the device management service manages the app using declarative management, look for the external-version-id key in the StatusAppManagedList status item that the device reports to the service.
If the device management service manages the app using MDM commands, look for the ExternalVersionIdentifier key in the Installed Application List command response that the device returns to the service.
Use the Apps and books metadata for organizations API to look up the externalVersionId key in the Apps.Attributes for the app.
Ask the app developer to provide the value.
The first 2 require access to an MDM and the third in order to use the Appsand books metadata you need to turn your account into an MDM vendor developer account.
So the final option is to "ask the app developer". I am the app developer and have no idea how to get this value. As a normal iOS developer is there no other way than to either contract an MDM or turn your account into an MDM vendor?