Swift Post Request - JSON Body is empty

I am trying to do a POST Request based on a REST server API.

Everytime I execute the code the server doesn't receive the sent body.The server receives a body looking like this: {}

The struct used in the request looks as follows:

struct CreateUser: Codable {   let first_name: String   let email: String   let password: String}

The Request is done as follows:

let userToRegister = CreateUser(firstName: "Max", email: "max.mustermann@gmail.com", password: "SECRET_PASSWORD")let str = API_ENDPOINTvar request : URLRequest = URLRequest(url: URL(string: str)!)let encoder = JSONEncoder()let jsonData = try? encoder.encode(userToRegister)request.httpBody = jsonDatarequest.httpMethod = "POST"return URLSession.shared.dataTask(with: request, completionHandler: {(data,response,error) in    DispatchQueue.main.async {        if let httpResponse = response as? HTTPURLResponse {            if httpResponse.statusCode == 200 {                //Code for successful request            } else {                //Code for failure in request            }        }    }})