Filling form data using a button

I am attempting to fill multiple form entries (mostly text boxes, some drop-downs) using a button.

The button is correctly resetting the hidden fields which generate the required random numbers, but attempting to change other field values is where it's breaking.

I'm using:

this.form.elements("Fel_Roll").value=Fel;

I've also tried

this.getField("Fel_Roll").value=Fel;

Fel is holding the random number and Fell_Roll is the field I want to have it input into IF the button is pressed.

I have also attempted it without the reset function, simply using the values currently in the (will be) hidden fields.