Can I add PayPal button on my landing page?

The usual procedure is that you use Genius Lander/PageDyno to create a landing page/squeeze page offering a free report or some such so you can capture your prospect's name and email in your autoresponder account. As soon as they signup you redirect them to your sales page with the pay button on it.

The first email in your autoresponder sequence contains details on how to access the free report or whatever you offered in exchange for their email address.