In order to add any item to the list of resources you would again use the blue info button.
Using Checks for filters
A special case are Checks that can be used in an "Apply to" filter. We use an example to demonstrate how that can be done.
This Fixup places some text on the first page of a PDF file.
The Check for "Page 1" is automatically added to the Script resources and in the params section there is an entry "apply_to" that references that resource. If you would have more than one Check in the resources you could select the one you want to use in your JS code.
The "isCheck" parameter serves a very special use case: It should almost always be "true". Only if the filter of the Fixup has predefined values (e.g. "All" or "Images") it may be set to "false".