Paypal Sandbox

<form target="paypal" 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="B7WZEHMFG3TTW">
<input type="image" src="https://www.sandbox.paypal.com/WEBSCR-640-20110401-1/en_US/i/btn/btn_cart_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/WEBSCR-640-20110401-1/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

Steps to follow for auto return:-
My Account
Profile
Profile Summary
Selling Preferences
Website Payment Preferences
Turning on auto return