CallCloudFlow.js

function callFlow(flowURL) {
    let parameter1 = "parameter1";
    let parameter2 = "parameter2";

    let body = {
        "Parameter1": parameter1,
        "Parameter2": parameter2
    };

    let req = new XMLHttpRequest();
    req.open("POST", flowURL, true);
    req.setRequestHeader("Content-Type", "application/json");
    req.onreadystatechange = function () {
        if (this.readyState === 4) {
            req.onreadystatechange = null;
            if (this.status === 200) {
                let resultJson = JSON.parse(this.response);
            } else {
                console.log(this.statusText);
            }
        }
    };
    req.send(JSON.stringify(body));
}