You can use a PayPal button creator to create a subscription button/link that your buyer can use to start a PayPal subscription.
Here is a rather complex button/link creator, but it works.
http://www.obrador.com/ipnwizard/ButtonCreator.htm
and this is PayPals own page about it.
https://www.paypal.com/us/cgi-bin/we...-intro-outside