How do I implement Manatee scanner in a PCL project?

I have a iOS/Android PCL project and want to implement Manatee scanner, but their samples don't cover PCL.

The sample provided for iOS:

scanButton.TouchUpInside += async (sender, e) => {

    //Make an instance of scanner
    var scanner = new MWBarcodeScanner.Scanner();

    //Call the scaner and wait for result
    var result =  await scanner.Scan();

    //If canceled, result is null
    if (result != null){
        new UIAlertView(result.type, result.code, null, "Close", null).Show();
    }

};

I am trying to convert it for my PCL project, and have an interface declared in the PCL project. In my iOS project I have this, but can't think how to complete it:

using System.Threading.Tasks;
using ManateeScanner.iOS.Helpers;
using Xamarin.Forms;
using MWBarcodeScanner;
using UIKit;

[assembly: Dependency(typeof(ManateeScanningService))]
namespace ManateeScanner.iOS.Helpers
{

    using iScan.PlatformDependency;


    public class ManateeScanningService : IManateeScanningService
    {
        public async Task<string> ScanAsync()
        {
            //Make an instance of scanner
            var scanner = new MWBarcodeScanner.Scanner();

            //Call the scaner and wait for result
            var result =  scanner.Scan();

            //If canceled, result is null
            if (result != null)
            {
                return result;
            }
        }