Hello,
Regrettably, this is not possible to accomplish. You will need to create a custom form control that could contain a drop-down list control with 
AutoPostback property set to 
true. 
When user changes the selected item, you can access other fields in the BizForm by getting their reference like this:
TextBox txtBox = (TextBox) Form.FieldControls["productprice"];and change their properies depending on the selected item. 
For more information about developing form controls can be found here: 
Developing form controlsBest regards,
Michal Legen