How to give custom function in D8 custom field widget class?
Because this is your custom widget, you don't need to create a custom submit function.

Just add WidgetBase::massageFormValues method to your class:

class CertfieldDefaultWidget extends WidgetBase {
  ....
  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
    // manipulate with $values where $values is an array of your fields defined in your formElement.
    return $values;
  }
  ....
}