How to force people to login to paypal using the email they provided on their checkout page?

[yourbesttrader]

1. I've seen some website where the website asks for your paypal email address during the order

2. Then when it goes to paypal checkout, it greys out the email address part of the login,

3. Which forces you to login using the same email address as you provided earlier on the main website.

How did they achieve this?

I've looked over the documentation and am a little lost.