Get/Set Multiselect Value from Client Script

I've got a multiselect field which displays transactions related to a case ("custevent_transactions") . Its set to display as a list.

I want to be able to add a value to this list via client script call to the following function which only seems to work in "edit" mode, not "create" mode.  Any ideas why not?:

 function AttachRma(values)  {      var id = values["id"];      if (!StringIsNullOrEmpty(id))      {          var thisRecordId = nlapiGetRecordId();            //THIS PART WORKS (EDIT MODE)          if (!StringIsNullOrEmpty(thisRecordId))          {              //DOM HACK TO ADD VALUE TO MACHINE AND REFRESH ... WORKS :-)              document.frames['server_commands'].document.location.replace('/app/common/custom/addrecord.nl?id=' + thisRecordId + '&custfield=CUSTEVENT_TRANSACTIONS&machine=multimachcustevent_transactions&recordid=' + id + '&action=add');              refreshmachine("multimachcustevent_transactions");          }            //THIS PART DOESN'T WORK (CREATE MODE)          else          {              var transactionIds = nlapiGetFieldValue("custevent_transactions");              var transactionIdsArr = new Array();              if (StringIsNullOrEmpty(transactionIds))              {                  transactionIdsArr = [id];              }              else              {                  transactionIdsArr = transactionIds.split(String.fromCharCode(5));                  transactionIdsArr.push(id);              }                            //THIS DOESN'T SEEM TO DO ANYTHING. WHY???              nlapiSetFieldValue("custevent_transactions", transactionIdsArr);          }      }  }    //FROM MY LIBRARY  function StringIsNullOrEmpty(value)  {        return value == null ||