Sharepoint 2010 : Show dialog box for custom "Add new item"

Javascript

  function OpenDialog(sPageURL) {

          options = SP.UI.$create_DialogOptions();

          options.width = 650;

          options.height = 650;

          options.url = sPageURL;

          options.dialogReturnValueCallback = Function.createDelegate(null, CloseCallback);

          SP.UI.ModalDialog.showModalDialog(options);

      }

      function CloseCallback(result, target) {

          location.reload(true);

      }

.aspx page

                  <tr>

                    <td class="ms-addnew" style="padding-bottom: 5px; width:100%">

                         <span style="height:10px;width:10px;position:relative;display:inline-block;overflow:hidden;" class="s4-clust"><img src="/_layouts/images/fgimg.png" alt="" style="left:-0px !important;top:-128px !important;position:absolute;" />

            

                         </span>&nbsp;<span id="addNewLink" runat="server"></span>

                         <a href="\" id="ahAddNew" runat="server" ></a>

                    </td>

                </tr>    

.cs page

      ahAddNew.HRef = "javascript:OpenDialog('" + oWeb.Url + "/_layouts/listform.aspx?PageType=8&ListId={" + oCalendarList.ID + "}&RootFolder=&ContentTypeId=" + oCalendarList.ContentTypes["Mission"].Id.ToString() + "')";

                    ahAddNew.InnerText = string.Empty;

                    ahAddNew.InnerText = "Add new mission";