You can install this plugin into your CakePHP application using composer.
The recommended way to install composer packages is:
composer require tgoeminne/referenceable
$this->addBehavior('Referenceable',[ 'field' => 'reference', ]);
Will add a reference number consisting of 2 letters and 6 digits between (111111 and 999999). Also make sure you create the field reference in your table.