SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3 - https://docs.sitefinity.com/track-a-custom-feather-form-widget-with-dec-track-form-submissions-server-side

ICustomSubmitFormEvent.cs

using Telerik.Sitefinity.Services.Events;

namespace SitefinityWebApp.Events
{
    interface ICustomSubmitFormEvent: IEvent
    {
        string Email { get; set; }
        string FirstMeal { get; set; }
        string SecondMeal { get; set; }
        string Dessert { get; set; }
    }
}