<---YOU DIV STARTS HERE FOR POPUP WINDOW-->
    <div id="iframeclasses">
    <?php add_thickbox(); ?>

    //Your form link

    <a  href="YOUR_LINK_GOES_HERE&KeepThis=true&TB_iframe=true" class="thickbox">
    </div>

<input type="submit" name="submit"  class="button button-primary emailbtn" value="<?php _e('YOUR_BUTON_TEXT', 'TEXTDOMAIN')?>" >
     </div>
    //your div close here