We have an Azure Entreprise agreement with a primary subscription to which a VSTS account is bound. We have installed a package management extension to host several useful packages for different projects. For each client, we create a subscription in this advisor and the associated VSTS account. We build our customer projects on the Hosted build agent of the latest subscription. But when the time came for the build process to restore the NuGet packages (using the Nuget installer task) that are stored in the main package, it looks like the build service cannot access this feed (probably because it is not part of the same same subscription).
Is there a way to allow an external build service in the permissions of the main channel, or even use the api key in the NuGet installer task to determine the client assembly in order to be able to restore packages from the main channel?
EDIT: I tried using a specific nuget.config file using apiKey (with the same api key that is locally installed on my dev machine) or packageSourceCredentials . None of 2 allows the build agent to restore packages.
EDIT: screenshot of agent linker (COETools.Testing comes from a custom feed on a different subscription) 
EDIT: nuget.config 
EDIT: I changed the build task to use the default agent provided by nuget.exe (I used it in the original control earlier) and I managed to return the packages from the nuget official feed. But I still cannot get one of the custom recovery feed.
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