How to pass user to Atlas function?

I am creating a delete authentication trigger. I would like to test the following code.

exports = function(authEvent) {
  const mongodb ="mongodb-atlas");
  const items = mongodb.db("realm-swift-template").collection("Item");
  const userId = authEvent.userId
  // items.delete({userId}) // what if deletion fails?

There is “Change User” button to select a user, but how do I pass this user to exports()?