sourcing Payments on SELECT field on custom form page

Hello,

I am creating a custom page in Suitescript 2.0 and would like the user to be able to select a payment record from a drop-down, but I don't know where to find out the internal id of the list of customer payments or even of transactions in general. I've tried 'customerpayment', but that seems to return just a list of all records. The code where I am trying to create this field is below.

        

   var pymtnum = form.addField({                  id: 'pymtnum',                  type: serverWidget.FieldType.SELECT,                  source: 'customerpayment',                  label: 'Payment Number'                                });              

Thanks!