jQuery Grid Plugin - jqGrid

Hi

I have grid which is populated based on selection from drop down box. It works fine but, I need to modify drop down to be multiselect so, I can choose more than one entry. How can it be done?

Thanks

<select name="prov_id" id="prov_id" multiple="multiple">
          <option value="25_3">agnico</option>
          <option value="24_3">agnico2</option>

</select> 

$myGrid.jqGrid(
   {
    
     url: myURL + '?method=GetTransactions', //CFC that will return the transactions
     postData: {
     prID: function() { return $("#prov_id option:selected").val(); },
     mn: function() { return $("#month option:selected").val(); },
     yr: function() { return $("#year option:selected").val(); }
    },
    //multiselect: true, // adds checkboxes in front of each line
    //multiboxonly: true,
    datatype: 'json', //We specify that the datatype we will be using will be JSON
    colNames:['Trx ID','Description','Cheque No','Amount', 'Debit','Credit', 'Month', 'Year', 'Status','Added by Employee'],

.............