#include <ArduinoJson.h>

constexpr uint8_t JSON_MEMORY {115};

StaticJsonDocument<JSON_MEMORY> doc;
char JSONmessageBuffer[JSON_MEMORY];

char JSONmessageBuffer2[JSON_MEMORY]  
{"{\"id\":\"req1\",\"src\":\"Bootsschuppen/Pumpe/resp\",\"method\":\"Switch.set\",\"params\":{\"id\":0,\"on\":true,\"toggle_after\":5}}"};

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

  JsonObject statement = doc.to<JsonObject>();
  statement["id"] = "req1";
  statement["src"] = "Bootsschuppen/Pumpe/resp";
  statement["method"] = "Switch.set";
  JsonObject params = statement.createNestedObject("params");
  params["id"] = 0;
  params["on"] = true;
  params["toggle_after"] = 5;

  serializeJsonPretty(doc, Serial );
  Serial.println();

  params["toggle_after"] = 6;
  serializeJson(doc, JSONmessageBuffer);
  Serial.println(JSONmessageBuffer);
  Serial.println(JSONmessageBuffer2);
}

void loop() {}