How to append filename with element from json?

Hi,
i`m trying the example pipeline named “test_JSON_Formatter”
Json generator → Mapper → Json Formater → File Writer

in Mapper i have:
Expression: _p_run_id.length != 0 ? $PAYLOAD.interface_id : $interface_id
Target path : $interface_id

in Json Formatter: i put nothing

In File Writer:

I want to append the filename with the content of the json element interface_id
i tried this c://temp//“test”+ $interface_id+“.xml”
but it returns error : interface_id is not defined
Where shoud i define the interface_id?
How snaplogic gets its content from the json?
Thank you