code: public SignUpRequest signUp(String email, String password, String connection) {
          Asserts.assertNotNull(email, "email");
          Asserts.assertNotNull(password, "password");
          Asserts.assertNotNull(connection, "connection");
  
          String url = baseUrl
                  .newBuilder()
                  .addPathSegment(PATH_DBCONNECTIONS)
                  .addPathSegment("signup")
                  .build()
                  .toString();
          CreateUserRequest request = new CreateUserRequest(client, url);
          request.addParameter(KEY_CLIENT_ID, clientId);
          request.addParameter(KEY_EMAIL, email);
          request.addParameter(KEY_PASSWORD, password);
          request.addParameter(KEY_CONNECTION, connection);
          return request;
      }
docs: Creates a sign up request with the given credentials and database connection.
  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", "topsecret", "db-connection")
  .setCustomFields(fields)
  .execute();
  } catch (Auth0Exception e) {
  //Something happened
  }
  }
  </pre>
  
  @param email      the desired user's email.
  @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.