code: public boolean onBackPressed() {
          if (subForm != null) {
              final boolean shouldDisplayPreviousForm = configuration.allowLogIn() || configuration.allowSignUp();
              if (shouldDisplayPreviousForm) {
                  resetHeaderTitle();
                  showSignUpTerms(subForm instanceof CustomFieldsFormView);
                  removeSubForm();
                  clearFocus();
                  return true;
              }
          }
  
          return formLayout != null && formLayout.onBackPressed();
      }
docs: Triggers the back action on the form.
  
  @return true if it was handled, false otherwise