Can I add an action button to a webinar or meeting?

Welcome to the Zoom Community, @KWeiss.

I’m not aware of any Zoom Webinar feature that would allow for a Call To Action button.

If you’re on a budget, you might take a look at WebinarTV, a free Webinar hosting service that does include a Call to Action button taking attendees to any URL you want. Check them out at https://WebinarTV.us. They don’t have a few of the bells and whistles Zoom has, but definitely worth considering, given your needs. 

Will a button is not available you can add donation information for all participants to see and use. You can leverage custom closed captions and APIs to ensure a message is pinned to the meeting or webinar display with the donation information (including a link) to direct attendees to a donation page. This will be visible throughout the meeting and let participants donate. If you are not a developer there is a Zoom app, Salepager, that lets you pin donation information as a caption to your Zoom meetings.