how to create a “Total Quantity” Field on Sales Order

Hi, i need to create a Field on the record “Sales Order” with the total of all the quantity of items lines in the order.

e.g.       

_______________

Item   quantity 

socks     3

gloves   2

__________________

total quantity  5

I tried with Scriptsuite but the value is null if the transaction  is added through an external flow (as an integration). it only works when a validation trigger works (for example, when I manually click “add” or “edit” in the item line)

/*
 * Client script function for Update TOT QTY
 */
function updateTotalQuantity(type, name) {
  //initialize variable for total quantity
  var totalQuantity = 0;
  
  // count number of lines in 'item' sublist
  var itemCount = nlapiGetLineItemCount('item');
  //for each line in the 'item' sublist, add value in quantity column to the total quantity variable
  for(var i=1; i<=itemCount; i++){
   lineLevelQuantity = nlapiGetLineItemValue('item', 'quantity', i)
   if(lineLevelQuantity != '' && lineLevelQuantity != null ) {
    totalQuantity += parseInt(lineLevelQuantity);
   }
  }
  
  //  assuming custbody_totalqty is the custom body field for the total quantity, change its value based the value from the computation above
  nlapiSetFieldValue('custbody_totalqty', totalQuantity, false);
 }

thanks