Dynamically binding in Formcalc

Hello,

please help with followting problem.

I need to change a binding of a text field (bedrate) dynamically depending of value of another field (FKART).

I can already change a caption of a field (bedrate) but not a value:

if ( xfa.form.data.#pageSet[0].Seite1.FKART.rawValue == "ZIFS") then

    bedrate.caption.value.#text = "Service Tax %";

    $ =?????

endif

How can I update binding for field (bedrate) to $.DATA.SUMS.SER_RATE?

Do I need to use "record" or smth else?

Thanks

Evgenij