How to refer details table controls on Master details Page

Hi,
I have master details page and I have loaded following code to "Java Script on Load" event and this is working fine.
var ctrlFrcode = Runner.getControl(pageid, 'Frequency_Code');

ctrlFrcode.setValue('Monthly')
But the same way how we can refer details table fields on that master details page so we can populate some values during.
Thanks

Sami