How do I make HTTP requests work synchronously so that the code is executed after a response is received and parsed.

So I am making a POST request to our own backend and the code looks something like this:

 // a method in a class for UI management
 public void SubmitUser(UserInfo user) {
         BackendAPI.CreateUser(user);
         CheckForErrors();
 }
 
 // the static method in BackendAPI class
 
     public static void CreateUser(UserInfo user) {
         Hashtable data = new Hashtable();
         data.Add("username", user.username);
 
         Request req = new Request("post", baseUrl + "users/", data);
         req.AddHeader("Authorization", authToken);
         req.Send((request) => {
             if (request.response.status == 422) {
                 MenuUIManager.usernameExists = true;
             } else if (request.response.status == 200) {
                 MenuUIManager.userCreated = true;
             }
         });
     }
 
 

So the CheckForErrors() method checks if the public bool variables of the instance of MenuUIManager is set depending on the response from the server.

The issue is that since it's in a callback the CheckForErrors() runs before the callback finishes executing.

So how can I make the CheckForErrors() only run once the callback inside CreateUser method completes?