Cannot create event in Windows C# Runtime Component

I am trying to create an event in Windows Runtime C# Component in this way,

namespace SampleNamespace
{
    public sealed class SampleClass
    {
        private static readonly Lazy<SampleClass> _instance = new Lazy<SampleClass>(() => new SampleClass());

        public static SampleClass Instance => _instance.Value;

        private SampleClass() { }

        public event EventHandler<string> SampleEvent;

        public void TestCode()
        {
            Debug.WriteLine("This is for test");
            SampleEvent?.Invoke(this, "value");
        }
    }
}

But when I try to build the project it gives this error,

error WME1026: Event add method 'SampleNamespace.SampleClass.add_SampleEvent(System.EventHandler<System.String>)' has return type 'System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken'. Event add methods must return System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken. Was the module that you are exporting compiled as a winmdmodule (/target:winmdobj) for Windows Metadata export?

I could not find anything to solve this issue. How can I solve it?