Modifying Paypal Button HTML

[molix97]

Hello everyone,
I have to add a thank you page to redirect people after the purchase.
I am trying to use this code: 

<input type="hidden" name="return" value="http://www.yourwebsite.com/successful.html">

The problem is that I am unable to access to the account and enable the return URL setting because I am doing it for a customer who gives me only the mail and then I am modifying the code with their variables. 
Do you have any solution?