code: def _handle_singular(cls, enterprise_customer, manage_learners_form):
          
          form_field_value = manage_learners_form.cleaned_data[ManageLearnersForm.Fields.EMAIL_OR_USERNAME]
          email = email_or_username__to__email(form_field_value)
          try:
              validate_email_to_link(email, form_field_value, ValidationMessages.INVALID_EMAIL_OR_USERNAME, True)
          except ValidationError as exc:
              manage_learners_form.add_error(ManageLearnersForm.Fields.EMAIL_OR_USERNAME, exc)
          else:
              EnterpriseCustomerUser.objects.link_user(enterprise_customer, email)
              return [email]
docs: Link single user by email or username.
  
          Arguments:
              enterprise_customer (EnterpriseCustomer): learners will be linked to this Enterprise Customer instance
              manage_learners_form (ManageLearnersForm): bound ManageLearners form instance