I am receiving an AuthenticationError.NOT_ADS_USER when using a ServiceAccount to access our MCC Account.

This is what I have in ads.properties:
api.adwords.jsonKeyFilePath=src/main/resources/JSONFILENAME.json api.adwords.developerToken=DEVELOPERTOKEN api.adwords.clientCustomerId=CUSTOMERID

This returns an accessToken, but the serviceAccountUser is null on the Credential from the Builder, created as follows:
Credential oAuth2Credential = new OfflineCredentials.Builder() .forApi(OfflineCredentials.Api.ADWORDS) .fromFile() .build() .generateCredential();

I've looked through the source code and don't see where it would pull serviceAccountUser from the properties file or JSON file if I were to add it there. How can I specify the user to impersonate when making the call? Is there some other reason this AuthenticationError would occur?

I'm creating this as an issue on the Java Lib, as other libs have the ability to specify.

Appreciate any support on this issue.

Thanks!
John