async in override context

Question

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<SLComposeSheetConfigurationItem[]> 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());
            ....
         }