Create interface and method in Forms.

public interface IShowAlert
{
     Task<bool> showAlert(string title, string message , string accept, string cancel);
 }

Inherit from the interface and implement the method in iOS project

 [assembly: Dependency(typeof(ShowAlert))]
 namespace PPPPCL.iOS
 {
     class ShowAlert : IShowAlert
     {
         public async Task<bool>  showAlert(string title, string message ,string accept, string cancel)
         {    
             var tcs = new TaskCompletionSource<bool>();

             UIAlertController vc = UIAlertController.Create(title,message, UIAlertControllerStyle.Alert);

             UIAlertAction action = UIAlertAction.Create(accept, UIAlertActionStyle.Default, (e) =>{ tcs.SetResult(true);});
             vc.AddAction(action);

             UIAlertAction action2 = UIAlertAction.Create(cancel, UIAlertActionStyle.Destructive, (e) => { tcs.SetResult(false);});
             vc.AddAction(action2);

             (UIApplication.SharedApplication.Delegate as AppDelegate).Window.RootViewController.PresentViewController(vc, true, null);

             return await tcs.Task;

         }
     }
 }

Call the method and get the value within callback in Forms .

bool result = await DependencyService.Get<IShowAlert>().showAlert("title","msg","ok","cancel");