NodeMCU handle post request - post

Question

I have a Arduino NodeMCU that needs to handle a Post request carrying an integer. How do I get it to handle a post request and handle the data? It sends a post request to example.com/Speed
Some Important code (condenced):
ESP8266WebServer server(80);
void handleRoot() {
digitalWrite(5, HIGH);
server.send(200, "text/html", "incert html");}
void setup() {
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
delay(500);
Serial.print(".");
}
Serial.println("");
Serial.println("WiFi connected");
Serial.println("IP address: ");
Serial.println(WiFi.localIP());
server.on ( "/", handleRoot );
server.on ("/save", handleSave);
server.begin();
server.on("/Speed", []() {
//Here is where I need it to get the data from the post request and save to int
myFunction(requestData);
Serial.println("Speed Change");
handleRoot();
});
}
void loop() {
server.handleClient();
}
Code sending reqest in JS:
var xhttp = new XMLHttpRequest();
xhttp.open('POST', '\\Speed', true);
xhttp.setRequestHeader('Content-Type', 'application/json');
xhttp.send(JSON.stringify({
value: slideAmount
}));