Drop down selection display a selection in another drop down list and enters a value in a field

I have 2 drop downs and a field that I am working with.

They are labeled as:

POS (this is dropdown 1 with an option labeled "JK POS Bundle")

PIN_PAD (this is dropdown 2 with an option labeled "PAX A35")

PINPAD_QTY (this is a blank field)

What I would like to do is the following:

When a user chooses the option "JK POS Bundle" from the POS dropdown list, the PIN_PAD dropdown list will show the option "PAX A35". I would also like it to populate the number "1" in the PINPAD_QTY field.

Below is what I have as a custom validation script in the POS dropdown. It is able to show "PAX A35" in the PIN_PAD dropdown list. I am unsure how to populate the number "1" in the PINPAD_QTY field at this point. Any assistance would be greatly appreciated.

var PIN_PADField = this.getField("PIN_PAD");

if (event.value=="JK POS Bundle" || event.value=="PAX A35")

    PIN_PADField.value = "PAX A35";

else PIN_PADField.value = PIN_PADField.defaultValue;