This is not Kentico restriction, this is ASP.NET restriction. :)
I think there is no way to solve this issue easy, without some javascript or C# coding.
Easiest way I see - use JavaScript. Something like this:
<form method="post" action="https://www.paypal.com/cgi-bin/webscr" target="paypal">
<input type="hidden" value="_s-xclick" name="cmd" /> <input type="hidden" value="6856762" name="hosted_button_id" /> <input type="image" border="0" alt="PayPal - The safer, easier way to pay online." name="submit" src="https://www.paypal.com/en_GB/i/btn/btn_cart_LG.gif" onclick="document.forms[0].action = 'https://www.paypal.com/cgi-bin/webscr';" /> <img width="1" height="1" border="0" src="https://www.paypal.com/en_GB/i/scr/pixel.gif" alt="" />
</form>
Comments:
1. We remove <form> tags, we use main form.
2. On the click to button we change form's action link, so all form will be submitted to paypal.
:)
But may be you can use simple <a> and submit parameters in query?
Like this
<a href="https://www.paypal.com/cgi-bin/webscr?_s-xclick=cmd&hosted_button_id=6856762" target="paypal">PayPal</a>