How to set the PO Number to a custom filed in the Po Line Item?

Hello,

I am very new to suitescripting and attempting to make a script that will set a PO Number to every line in the Items Subtab in the Purchase Order.

First of all, I am not sure if this should be a user event script or the client script. We would actualy like to deploy it only on a particular custom PO Form.

Here is my code:


function setLineItemValue()
{
var currentRecord;
var trannumber; 

// Execute this only when Purchase order is created, viewed or edited.
if( ( type == 'create') || (type=='edit') || (type=='view'))
{  
  currentRecord = nlapiGetRecord(); 
  // Get the PO number before submit
  trannumber = currentRecord.getFieldValue('tranid');