Connect to private nuget feed using Nuget.protocol

I'm using a private NuGet feed through Azure DevOps.To see information about a public NuGet package from code, one simply has to follow the documentation here. In particular:

var logger = NullLogger.Instance;var cancellationToken = CancellationToken.None;var cache = new SourceCacheContext();    var source = new PackageSource("*my feed*/nuget/v3/index.json");source.Credentials = new PackageSourceCredential("*my feed*/nuget/v3/index.json", "tokenName", "token", false, null);var repository = Repository.Factory.GetCoreV2(source);var resource = await repository.GetResourceAsync<PackageMetadataResource>();IEnumerable<IPackageSearchMetadata> searchMetadata = await resource.GetMetadataAsync("My.Package.Name",    includePrerelease: false,    includeUnlisted: false,    cache,    logger,    cancellationToken);

I cannot get through step

await repository.GetResourceAsync<PackageMetadataResource>()

which keeps failing with a

"Unable to load the service index for source *my feed*/nuget/v3/index.json."

How am I expected to identify an Azure DevOps feed from code, to be able to get the IPackageMetadata information?

Note:I have followed the answers here