hmmm… Well Plesk is a host control panel, but basically if you want to use the theme’s own pricing tables with the button, you will need to find out how to link the buttons then. I recommend you contact their support and let them know what you want to do and see if they have a solution for you. Tell them that you simply want to add a “buy” link to a custom button. If they have documentation, also check that and see if there’s any solutions as well.
The code you shown above, does that not work?