$query = $this->createQuery();                                   
$query = $query->statement(
    //manually creating PreparedStatement should not be necessary
    //anymore in typo3 v8+
    new \TYPO3\CMS\Core\Database\PreparedStatement(              
        'SELECT * FROM tablename WHERE col = ? LIMIT 1',
        'tablename'
    ),
    ['myvalue']
);
$result = $query->execute();