SingleLineTextWithPlaceholderField.cs

using Sitecore.Form.Core.Attributes;
using System;
using System.ComponentModel;
using System.Web.UI;

namespace CustomWFFM.CustomFieldTypes
{
  [ValidationProperty("Text")]
  public class SingleLineTextWithPlaceholderField : Sitecore.Form.Web.UI.Controls.SingleLineText
  {
    [VisualCategory("Custom Properties")]
    [VisualProperty("Placeholder Text", 2)]
    [DefaultValue("")]
    public string PlaceholderText { get; set; }

    protected override void OnInit(EventArgs e)
    {
      if (!string.IsNullOrEmpty(PlaceholderText))
      {
        textbox.Attributes["placeholder"] = PlaceholderText;
      }

      base.OnInit(e);
    }
  }
}