$(function () {
    $("#btn").click(function () {
      var usercode = $("#UserCode").val();
      var verifycode = $("#Verifycode").val();
      if ($.trim(usercode) == "" || $.trim(verifycode) == "") {
        alert(" 您的[客户号]或者[验证码]必须填写！");
        return false;
      }
      else {
        document.SearchForm.action = '/index/Search?u=' + usercode;
        document.SearchForm.submit();
      }
    });
  });

<form id="SearchForm" name="SearchForm" method="post">
 ……
</form>