Just found it.
Needed to modify the code to:
output.AddAttribute(HtmlTextWriterAttribute.Type, "text");
if (!this.IsEnabled)
{
output.AddAttribute(HtmlTextWriterAttribute.Disabled, "disabled");
}
output.AddAttribute(HtmlTextWriterAttribute.Value, this.DisplayName);
output.RenderBeginTag(HtmlTextWriterTag.Input);
output.RenderEndTag();