How to write xhr cross domain post for livetex chat?

Question

JavaScript

Greetings.
The first time I encountered cross-domain requests and I still can’t parse them. There is a task on the chat api livetex.github.io/messaging-api to build a chat. When I try to send a message, I get a response

Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '*******' is therefore not allowed access. The response had HTTP status code 404.

$.ajax({
            url: this.urlGen(),
            type: "POST",
            data : this.msgGen(),
            // contentType: 'application/json',
            headers: {
                "Access-Control-Allow-Origin": "*"
            },
            dataType: 'json',
            success: function (response) {
                workForm.switch()
                console.log('Success',response);
            },
            error : function (response) {
                console.error('Error',response);
            }
        });

Answer 1

Access-Control-Allow-Origin must be set on the server, i.e. at livetex.github.io/messaging-api