Sandbox integration with Paypal on asp.net using the Website Payment Standard

I have an integrated paypal sandbox in my asp.net using the Website Payment Standard as shown below.

<form name="paypal" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick" />
                            <input type="hidden" name="business" value="abc_131556943_biz@gmail.com" />

                            <input type="hidden" name="item_name" value="MyItem" />

                            <input type="hidden" name="amount" value="0.01" />

                            <input type="hidden" name="return" value="http://stackoverflow.com/questions/ask" />
                            <input type="hidden" name="cancel_return" value="http://stackoverflow.com" />
                            <input type="hidden" name="notify_url" value="http://stackoverflow.com" />
                            <input type="hidden" name="no_shipping" value="1" />
                            <input type="submit" value="Please Wait....." style="visibility: hidden" />
                            </form>

Now the problem is that when I redirect to paypal, it first asks for a login in the paypal sandbox account. I want to ask what is required to enter paypal sandbox for payment. Is there any parameter that we can send in a hidden field so that it does not ask for entry to the sandbox.

Thank you Rajbir