c# - async in override context

I am working on an iOS Xamarin Project. I need to create a
share extension.

In a share extension, we can
override this method:

 public
            override SLComposeSheetConfigurationItem[]
            GetConfigurationItems()

I
want to call async methods in this method.
So i need to put an async keyword
like this:

 public override async
            Task
            GetConfigurationItems()

But
C# compiler gives me an error, because override initial method has not this signature.
So i do not know how to
do...

Thanks

**
EDIT **

Here is full
example

 public override
            SLComposeSheetConfigurationItem[] GetConfigurationItems()
 {
 // To
            add configuration options via table cells at the bottom of the sheet, return an array of
            SLComposeSheetConfigurationItem here.


 var result =
            await service.make_computation());

 ....

            }