CustomerLookup.cs

        <div class="field-row">
            <div class="field-box">
                <%=Html.AutoCompleteFor(
                    x => x.Customer.CompanyName,
                    x => x.Customer.Id,
                    Url.Action("Autocomplete", "Customer")
                    ) %><br />
            </div>
        </div>

HtmlExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Html;

namespace Demo.WebSite.Extensions
{
    public static class HtmlExtensions
    {
        private static object readonlyAttribs = new { disabled = true };
        
        static HtmlExtensions()
        {
        
        }

        public static MvcHtmlString ReadonlyTextBoxFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
        {
            return htmlHelper.TextBoxFor(expression, readonlyAttribs);
        }

        public static MvcHtmlString ReadonlyTextAreaFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
        {
            return htmlHelper.TextAreaFor(expression, readonlyAttribs);
        }
        
        public static MvcHtmlString DatePickerFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
        {
            return htmlHelper.EditorFor(expression);
        }

        public static MvcHtmlString AutoCompleteFor<TModel, TDisplayProperty, TKeyProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TDisplayProperty>> displayField, Expression<Func<TModel, TKeyProperty>> keyField, string url)
        {
            string displayFieldName = HtmlHelper.GenerateIdFromName(ExpressionHelper.GetExpressionText(displayField));
            string keyFieldName = HtmlHelper.GenerateIdFromName(ExpressionHelper.GetExpressionText(keyField));

            StringBuilder sb = new StringBuilder();
            sb.Append(htmlHelper.EditorFor(displayField));
            sb.Append(htmlHelper.HiddenFor(keyField));
            sb.AppendLine();
            sb.AppendLine("<script type=\"text/javascript\">");
            sb.AppendLine("$(function(){");
            sb.AppendLine("$('#" + displayFieldName + "').autocomplete({");

            sb.AppendLine("source:  function(request, response){");
            sb.AppendLine("$.getJSON('"+url+"', request, response);");
            sb.AppendLine("}");
            
            sb.AppendLine(",minLength: 1");

            sb.AppendLine(",select: function(event, ui) {");
            sb.AppendLine("$('#" + keyFieldName + "').val(ui.item ? ui.item.id : '');");
            sb.AppendLine("}");

            sb.AppendLine("});");
            sb.AppendLine("});");
            sb.AppendLine("</script>");

            return MvcHtmlString.Create(sb.ToString());
        }
    }
}