<form name="addForm" action="asd.php" method="post">
<input type="text" name="link" value="http://link"><input type="submit" name="go" value="Показать">
</form>
<?php
if (isset($_POST['link'])) echo "[ url=mysite.com][ center][ img]".$_POST['link']."[ /img][ /center][ /url]";
?>