// get the id of the sales order where item fulfillment is created from
        var soid = <id of the sales order>;

        // set the internal id of the financial location custom segment
        var sofl = <internal id of the custom segment>;

        // get the value of the financial location from the sales order
        var financial_location = nlapiLookupField('salesorder', soid, sofl);

        // load the item fulfillment record
        var itemfulfillment = nlapiLoadRecord('itemfulfillment', nlapiGetRecordId());

        // set the value of the financial location from the Sales Order to the Item Fulfillment
        itemfulfillment.setFieldValue(sofl, financial_location);

        // Save the record
        nlapiSubmitRecord(itemfulfillment);