SF_14.3, SF_15.0 - https://www.progress.com/documentation/sitefinity-cms/for-developers-extend-forms-connectors-definitions

ExtendFormConnector.cs

using System;
using Telerik.Sitefinity.Configuration;
using Telerik.Sitefinity.Modules.Forms;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.UI.Fields;
using Telerik.Sitefinity.Web.UI.Fields.Config;
using Telerik.Sitefinity.Web.UI.Fields.Enums;

namespace SitefinityWebApp
{
    public class ExtendFormConnector : FormsConnectorDefinitionsExtender
    {
        public override void AddConnectorSettings(ConfigElementDictionary<string, FieldDefinitionElement> sectionFields)
        {
            var myTextField = new TextFieldDefinitionElement(sectionFields)
            {
                Title = "myTextField",
                DataFieldName = string.Format("{0}.{1}", "Attributes", "myFieldName"),
                DisplayMode = FieldDisplayMode.Write,
                FieldName = "myFieldName",
                FieldType = typeof(TextField),
                ID = "myId",
                ResourceClassId = "myClass",
            };

            sectionFields.Add(myTextField);
        }

        public override int Ordinal
        {
            get { return 1; }
        }

        public override string ConnectorName => "MyConnectorName";

        public override string ConnectorTitle => "MyConnectorTitle";

        public override string SectionTitle => "MyConnectorSectionTitle";
    }
}