public FormBuilder::submitForm($form_arg, FormStateInterface &$form_state)
Retrieves, populates, and processes a form.
This function allows you to supply values for form elements and submit a form for processing. Compare to self::getForm(), which also builds and processes a form, but does not allow you to supply values.
There is no return value, but you can check to see if there are errors by calling $form_state->getErrors().
// register a new user
$form_state = new FormState();
$values[