ajax call back function i am using drupal 7 

i am trying to add a button within form and successful.  now i want to call a function when someone click on that button 

here is my code. 

function clicktell_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    //print_r($form);

    if($form_id=='user_register_form') {
        $form['add_button'] = array(
            '#type' =&gt; 'button',
            '#value' =&gt; 'Verifynow',
            '#ajax'&gt; array (
                'callback' =&gt; '_udr_ajax_handler',
                'progress' =&gt; FALSE,
            ),
        );


    }
}

function _udr_ajax_handler($form, &amp;$form_state) {
    echo "hello "; exit;
}


But this is not working fine. 
Does anybody tell me how to solve this 
Thanks
You missed equal sign '=' near '#ajax'.