require_once("includes/config.php")
, add a new variable like
$random =rand(10000,999999);
This will create a random number between 10000 and 999999.
You can also add some random letters for uniqueness. Then search for sendMail($adminMail in that file, and above the “found” string, you will see the $data variable with various values. Edit it by adding another line like that and use $random variable as value.
The text on the left side is the short code which can be used in emailTemplates/timeBookingConfirmationAdmin.php (timeBookingConfirmationCustomer.php, eventBookingConfirmationAdmin.php, eventBookingConfirmationCustomer.php) to send that variable to admin/customer.