I am using Paypal Express Checkout api-version 109. Unlike in paypal standard, there is no option in to pre-populate name, address, email, phonenumber, zip , country etc. on the landing page of Paypal. I went through the API page of SETEXPRESSCHECKOUT, but didn't find any relevent parameters. Are there any parameters to do that as I might have missed ?
If people are paying with a PayPal account then their details are already available in the PayPal account. No need to pass anything.
If you do want to pass details to the checkout you can include a shipping address in the SetExpressCheckout request and then also include the ADDROVERRIDE parameter set to true. That will make it so that the system uses the address you send and does not allow the user to change it once they're signed in.
If you're talking about pre-populating the Guest Checkout (credit card) form that is displayed on the Express Checkout page, though, you cannot pass data to that. It cannot be pre-filled.