Having problem with automatically populate look up text field

I have been looking and looking through these forums for an answer on this but so far have not found anything that will work. I want to be able to make a selection from a dropdown and have it fill in the text field below it automatically with the value pulled from that record.

I have one form called Class_List which holds a bunch of classes and their prices. The class name field is 'Class' and the price field is 'Price'. I have Student_Registration_Form where students can sign up for a class. They select a program. When they select their program, it changes the class list for the dropdown below (field 'Class') (that works fine). Then when they choose their class, I want the price field below to automatically populate with the correct price from Class_List.

Based on another post I read, here is what I have so far:

in "On User Input" I have:

if (input.Class != "" )
{ myClass = Class_List[Name == input.Class];
input.Price = myClass.Price;

}

When I save the script I get the following error:
Error at line:0
Form Class_List does not exist in this application

How do I get a lookup text field to populate automatically?

Thanks