To disable synchronization context you should call the ConfigureAwait method:

async Task() Foo()
{
    await Task.Run(() => Console.WriteLine("Test"));
}

. . .

Foo().ConfigureAwait(false);

ConfigureAwait provides a means to avoid the default SynchronizationContext capturing behavior; passing false for the flowContext parameter prevents the SynchronizationContext from being used to resume execution after the await.

Quote from It’s All About the SynchronizationContext.