重定向到PayPal Express Checkout时自动填写电子邮件帐户

I'm implementing PayPal's ExpressCheckout with the PayPal-PHP-SDK and I would like to know if it's possible to add a field in the payment request so the user's email account could be filled in when he/she is redirected to the PayPal site where the login screen is displayed.

Here's a screenshot of the field I'm talking about:

Thanks in advance