For creating the Paypal Button link you have to go first to the Button Manager API.
When the button is created you can copy the link from the email tab.
<a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=MJ7V43GU2H386"
target="_blank">
<img src="https://www.paypalobjects.com/en_US/GB/i/btn/btn_donateCC_LG.gif" alt="PayPal this"
title="PayPal – The safer, easier way to pay online!" border="0" />