When you buy a tool for your development, sometimes that tool comes via a private NuGet server which often requires a username and password.
In order to access the server, you’ll need to add these credentials to VSTS. To do this, click the Gear, then Services.
Click New Service Endpoint, then choose NuGet from the list that appears.
On the dialog that appears, select Basic Authentication and enter the details for the external NuGet server.
Now go to your build definition, and choose the NuGet restore build setup you have already added there. Under Feeds and authentication, choose Feeds in my NuGet.config and enter the path to your nuget.config in the solution.
Then choose the credentials you just added in the following box.