Form within thickbox

When I try and insert a form into a thickbox, it seems to be removed completely. What do I need to change?

<?php
    // Enqueue thickbox js and css
    add_thickbox();
?>

<div style="text-align:center;padding:20px 0;"> 
<input alt="#TB_inline?width=500&height=600&inlineId=examplePopup1" title="Insert/edit link" class="thickbox" type="button" value="Tickbox 1" />  
</div>

<div id="examplePopup1" style="display: none">
    <div name="buffer">
        <form name="test" action="www.test.php" method="post">
            <input type="text" class="item-permalink" value="test">
            <input type="submit" value="Add Link" class="button button-primary" id="link_form-submit" name="link_form-submit">
        </form>
    </div>
</div>