code: public SignUpRequest signUp(String email, String username, String password, String connection) {
          Asserts.assertNotNull(username, "username");
  
          CreateUserRequest request = (CreateUserRequest) this.signUp(email, password, connection);
          request.addParameter(KEY_USERNAME, username);
          return request;
      }
docs: Creates a sign up request with the given credentials and database connection.
  "Requires Username" option must be turned on in the Connection's configuration first.
  i.e.:
  <pre>
  {@code
  AuthAPI auth = new AuthAPI("me.auth0.com", "B3c6RYhk1v9SbIJcRIOwu62gIUGsnze", "2679NfkaBn62e6w5E8zNEzjr-yWfkaBne");
  try {
  Map<String, String> fields = new HashMap<String, String>();
  fields.put("age", "25);
  fields.put("city", "Buenos Aires");
  auth.signUp("me@auth0.com", "myself", "topsecret", "db-connection")
  .setCustomFields(fields)
  .execute();
  } catch (Auth0Exception e) {
  //Something happened
  }
  }
  </pre>
  
  @param email      the desired user's email.
  @param username   the desired user's username.
  @param password   the desired user's password.
  @param connection the database connection where the user is going to be created.
  @return a Request to configure and execute.