Select Hidden from the Display Class menu.
The Hidden display class corresponds to the <input type=hidden’/> HTML component. This component supports only single-valued data types because there is no way to reliably serialize and deserialize multi-valued data types.
If you have a List that you want to render it as a string, you must explicitly convert it to a String, as shown in the following example:
<Field name=’testHiddenFieldList’ > <Display class=’Hidden’/ > <Derivation> <invoke name=’toString’> <List> <String>aaaa</String> <String>bbbb</String> </List> </invoke> </Derivation> </Field>