You will have to create a custom finder method
in your user table you must add something like this:
public function findAuth(\Cake\ORM\Query $query, array $options)
->select(['id', 'username', 'password'])
->where(['Users.active' => 1])
and add this method to your finder
'authenticate' => [
'Form' => [
'finder' => 'auth'
'auth' been the name of your finder method.