Ad Exchange Seller REST API and API Key

[ALTAY KARAKUŞ]

Hi,
I am trying to access Ad Exchange Seller REST API with API key with C#, using Google.Apis.AdExchangeSeller.v2_0.

But getting following error;

Google.GoogleApiException: Google.Apis.Requests.RequestError
Login Required [401]

Is there way that I can access to Ad Exchange Seller REST API without needing user interaction.

My code is as simple as following:

var service = new AdExchangeSellerService(
new BaseClientService.Initializer
{
ApplicationName = "Advoly Client",
ApiKey ="[MY API KEY]",
}
);

var accounts = service.Accounts.List().Execute();