Create fake message for EventSource in cypress

I am trying to mock SSEs in cypress. There are a few questions on SO, and a few issues on GitHub about this, but nothing definitive, or particularly relevant to my setup. I feel like this could be simple, if I can "fake" a message on an EventSource.

My Setup

I have an EventSource that connects to an SSE endpoint on our server:

const eventSrc = new EventSource(`/api/sseEndpoint`, options);

This gets fed to a subscription function within a redux-saga:

const chan = yield call(subscribe, eventSrc);

const message = yield take(chan)

// pass message to further action/reducer processing:
yield handleMessage(message) 

Where the subscribe funcrion is pretty simple:

export function subscribe(eventSrc: EventSource): EventChannel<unknown> {
  const subscriptions: Subscribe<unknown> = emitter => {
    eventSrc.onmessage = (msg): void => {
      emitter(msg);
    };
    eventSrc.onerror = fn,
    eventSrc.onopen = fn;
    return (): void => {
      eventSrc.close();
    };
  };
  return eventChannel(subscriptions);
}

Where eventChannel is part of the redux-saga channel api.

In need to be able to spoof a message to this EventSource from within cypress. I am trying to figure out what I can attach to the cypress window in the following fashion:

if (window.Cypress) {
  window.eventSrc = eventSrc;
}

Such that from within cypress commands, I can create a command to hijack that window.eventSrc to dispatch a message:

Cypress.Commands.add("sendSSE", sse => {
  cy.window()
    .its("eventSrc")
    .then(eventSrc => {
      eventSrc.dispatchEvent({
        data: JSON.stringify(sse),
        lastEventId: "",
        origin: "http://localhost:3000",
        type: "message"
      });
    });
});

Dispatching / spoofing an event?

As you can see, I've tried using the dispatchEvent function which is on the prototype of an EventSource. Unfortunately, I get the error message from within the cypress test:

Saga or channel was provided with an undefined action
Hints:
  - check that your Action Creator returns a non-undefined value
  - if the Saga was started using runSaga, check that your subscribe source provides the action to its listeners

I must not be undertanding the dispatchEvent function. Is there a way to use the EventSource that I've attached to the window object to send an event programmatically from within cypress, such that the eventSrc.onMessage = (msg) => { ... } function will fire?