I'm not sure, but it's possible that text editor in the form builder is limiting the type of html you can use. In any case, you shouldn't place the button on the form itself since it will redirect your visitor to the PayPal site and the form will be lost.
We have a PayPal connector on our professional plan ($34/month) that will allow you to do what you need. If you don't want to upgrade your account, I'd suggest you place the button on your "thank-you" page. You won't be able to track what option they chose though.