How to combine TaskCompletionSource and CancellationTokenSource?

If I understand you correctly, you can do it like this:

using (cancellationToken.Register(() => {
    // this callback will be executed when token is cancelled
    task_comletion_source.TrySetCanceled();
})) {
    // ...
    await task_comletion_source.Task;
}

Note that it will throw an exception on your await, which you have to handle.