class somePlugin extends Plugin {

    public $customerId = '1234';

    public function init() {

        parent::init();
        self::$plugin = $this;

        Event::on( Order::class, Order::EVENT_BEFORE_COMPLETE_ORDER, function( Event $e ) {
            $order             = $e->sender;
            $customerId        = $this->getCustomerId();

            // on every page reload it does get reset to the default var, what is not wanted
            $order->customerId = $customerId;
        } );
    }

    public function setUser( $userId ) {

        self::$plugin = $this;

        // get userModel by userId
        $userModel = Craft::$app->users->getUserById( $userId );

        // check if  commerce record for craft user exists
        if ( ! $this->customers->getCustomerByUserId( $userId ) ) {

            // create and save new customer
            $newCustomer       = new Customer();
            $newCustomer->user = $userModel;
            $this->customers->saveCustomer( $newCustomer, false );
        }

        // store customerId
        $customerId = $this->customers->getCustomerByUserId( $userId )->id;

       // sets the new and wanted it wich gets overwritten by page reload
        $this->setCustomerId( $customerId );

    }

    public function setCustomerId( $customerId ) {
        $this->customerId = $customerId;
    }

    public function getCustomerId() {
        return $this->customerId;
    }
}