Donation button on webinar registration page

Hello all, 

My nonprofit is hosting a series of Zoom webinars.  I've set up and branded (logos, speakers) the registration page; however, I cannot determine how to add an optional Donation button to the registration page (we are not charging for the webinars). I have seen other organizations' registration pages with this feature.  Our PayPal appears to be connected to Zoom via POWR, so far as I can tell, but I see no option anywhere to add a button.  Thanks.