In a lot of cases you need to find a field id for a field. The field id is an unique number that Ragic assigns to each field that you created. It gives your program an unambiguous way to refer to a field. With this design, fields can even have the same name under a sheet and Ragic will still be able to distinguish between them.
Go to the design mode, and click on the field you want to reference to. On the left sidebar, you're going to see the Field Name. The Field Id is right under the field name.