http error 400 with esp8266 when trying to send a JSON objekt

Question

Hej I wish to send a json objekt from my esp8266 to a SQL database which works when i test it with postman. The problem occurs when I try sending it from esp8266 then i receive a http 400 error, but I dont understand why because i used the same objekt that I used in Postman and I also use the same URL namely http://daweatech.dk/api/measurements. My code can be seen below,

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include <WiFiClient.h>
#include <ArduinoJson.h>

const char* ssid = "Pedersensnet";
const char* password = "";
String output;

//Your Domain name with URL path or IP address with path
const char* serverName = "http://daweatech.dk/api/measurements";


void setup() {
  Serial.begin(115200);

  WiFi.begin(ssid, password);
  Serial.println("Connecting");
  while(WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.print("Connected to WiFi network with IP Address: ");
  Serial.println(WiFi.localIP());
 

output = "{\"timeStart\":\"2022-05-03T09:00:00\",\"timeEnd\":\"2022-05-03T10:00:00\",\"unit\":\"kWh\",\"measurement\":0.024,\"energyType\":\"Electric\",\"meterId\":\"123456789\"}";
Serial.println(output);

      WiFiClient client;
      HTTPClient http;
      
      // Your Domain name with URL path or IP address with path
      http.begin(client, serverName);

      
      // If you need an HTTP request with a content type: application/json, use the following:
      http.addHeader("Content-Type", "application/json");
      int httpResponseCode = http.POST(output);

      // If you need an HTTP request with a content type: text/plain
      //http.addHeader("Content-Type", "text/plain");
      //int httpResponseCode = http.POST("Hello, World!");
     
      Serial.print("HTTP Response code: ");
      Serial.println(httpResponseCode);
        
      // Free resources
      http.end();

}

void loop() {
 
   
}