I'm trying to establish SSE connection and do some long-taking actions on server-side, informing user about how it goes through SSE events.

Actually, I don't understand how would I send some data along with new connection. I have to combine regular ajax with new EventSource or there is a way to transfer post data inside that event?

Here is what I have so far, and I need to send pretty big JSON with the request. Is it possible or the only way to send data is GET?

var source = new EventSource('/terminal/ajax-put-packet-trade-order/');