Silverlight and Paypal Adaptive Payments

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PayPal

Silverlight4

I have a Silverlight 4 LOB application to which I would like to add a simple subscription service. Have set up a secure WCF service which returns a valid paykey. The question is how to utilise this resource in Silverlight without exposing the key on the client.

A simple solution that works but exposes the key on the client uses HtmlPage.Window.Navigate(new System.Uri(redirectURL)) on a SL button click event to mimic the call to response.redirect(redirectURL) in the asp samples.

I have also seen working samples of Express Checkout using the Merchant button scripts inside a div element in default.aspx as follows.

<div id="paypal_area" style="position:absolute;top:0px;left:0px;display:none">
    <form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
    <input type="hidden" name="cmd" value="_s-xclick">
    <input type="hidden" name="hosted_button_id" value="XXXXXXXXXX">
    <input type="image" src="https://www.sandbox.paypal.com/en_US/i/btn/btn_subscribeCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
    <img alt="" border="0" src="https://www.sandbox.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
    </form>
</div>

My question relates to the current best approach in Silverlight to using the Adaptive Payments API. I assume that I need a script in default.aspx that responds to a html button click event but I don't know how to implement the javascript required or how to position the button. Any help would be appreciated.

Regards,
Justin