Call external function from expression

I'm use the example file "Loading script & WASM" from construct 3. In the scriptfile "external.js" i'm add a new function:

function testing() { return "ok it works!"; }

How can i call in a expression the function testing in script external.js and get the callback "ok it works!" ?

Create an event sheet function with a string return value. Inside the event sheet function, add a script action with:

runtime.setReturnValue(testing());