WLED JSON API

Well It's been very instructive and I have the wled script running in Max ! but what should be the next step ? My first raw of questions : In the script there's is this line: xhr.open("POST", "http://" + ip + "/json", true); I assume that in this line "http://" should be the address of the API, and " + ip +" the IP address of the device, but what is "/json" ? Second raw of questions : I don't understand how to send post requests now... here is my patch and the script to access the json API

// Define the function to send a command to WLED
function sendWLEDCommand(ip, data) {
    var xhr = new XMLHttpRequest();
    xhr.open("POST", "http://" + ip + "/json", true);
    xhr.setRequestHeader("Content-Type", "application/json");
    xhr.send(JSON.stringify(data));
}

// Functions that can be called from Max
function changePalette(ip, paletteID) {
    sendWLEDCommand(ip, {"seg": [{"pal": paletteID}]});
}

function changeEffect(ip, effectID) {
    sendWLEDCommand(ip, {"seg": [{"fx": effectID}]});
}

function changeSpeed(ip, speed) {
    sendWLEDCommand(ip, {"seg": [{"sx": speed}]});
}

function changeIntensity(ip, intensity) {
    sendWLEDCommand(ip, {"seg": [{"ix": intensity}]});
}