How to define variable in a signal event?

Greetings,

To configure a signal event, select the element in the diagram, go to the Details panel, General tab, General pane, and specify the content of the signal in the Signal field. The signal content is a string. Either enter the string in the form field, or select it from the list of signals already defined.You can also defined variables at an intermediate catch signal event.

Based on the previous information, can someone please tell me how the variable is defined in a signal event?

I have tried defining the process variable in an intermediary event like $caseID, {{caseID}}, ${caseID}, `${caseID}` but it doesn't work.

Thanks in advance.