Where is the OnCompleted callback for Task?

I'd like to be be able to do this:

class MyClass
{
    IRepo repo;
    public MyClass(IMyRepo repo) { this.repo = repo; }

    long ID { get; set; }
    string Prop1 { get; set; }
    string Prop2 { get; set; }

    public async Task LoadAsync()
    {
        await Task.WhenAll(
            repo.GetProp1ByIDAsync(ID).OnComplete(x => Prop1 = x),
            repo.GetProp2ByIDAsync(ID).OnComplete(x => Prop2 = x)
        );
    }
}

Of course, I can't seem to find any standard library that has this OnComplete extension for Task. Do I really need to create my own or is there a library that already has this extension method? I see that there's ContinueWith but that does not give me the unwrapped result, I still have to await it or .Result it... so wouldn't that block the thread? Holding up the second repo call until this completed? If it doesn't hold it up then why am I getting that result still wrapped, would it be cleaner to return the unwrapped result to me? Or am I missing something?