Technically it's already there in the link. If you want to print it out you'll need to change lib/Email/Messages/ReservationEmailMessage.php starting on line 74 to this:
protected function PopulateTemplate()
{
$currentInstance = $this->reservationSeries->CurrentInstance();
$this->Set('StartDate', $currentInstance->StartDate()->ToTimezone($this->timezone));
$this->Set('EndDate', $currentInstance->EndDate()->ToTimezone($this->timezone));
$this->Set('ResourceName', $this->reservationSeries->Resource()->GetName());
$this->Set('Title', $this->reservationSeries->Title());
$this->Set('Description', $this->reservationSeries->Description());
$repeatDates = array();
if ($this->reservationSeries->IsRecurring())
{
foreach ($this->reservationSeries->Instances() as $repeated)
{
$repeatDates[] = $repeated->StartDate()->ToTimezone($this->timezone);
}
}
$this->Set('ReferenceNumber', $currentInstance->ReferenceNumber());
$this->Set('RepeatDates', $repeatDates);
$this->Set('RequiresApproval', $this->reservationSeries->RequiresApproval());
$this->Set('ReservationUrl', sprintf("%s?%s=%s", Pages::RESERVATION, QueryStringKeys::REFERENCE_NUMBER, $currentInstance->ReferenceNumber()));
$this->Set('ICalUrl', sprintf("export/%s?%s=%s", Pages::CALENDAR_EXPORT, QueryStringKeys::REFERENCE_NUMBER, $currentInstance->ReferenceNumber()));
}
Then change /lang/en_us/ReservationCreate.tpl to display the ReferenceNumber wherever you want. You'll probably need to clear your template cache, either through the Server Settings while logged in as admin, or by deleting the files in /tpl_c