Good evening Patrick and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Is it possible to add an additional button with external link in the event calendar overview?
Absolutely. In fact, you can customize any view to your liking.
You might want to read our Themer’s guide to get a sense of how that works.
Basically, you edit the view’s code, add code for your button and upload the modified file back in your WordPress theme’s folder.
On a related note, you might be interested in the following: https://theeventscalendar.com/knowledgebase/make-links-event-go-straight-event-website/
Let me know if that helps.
Have a great day!
Geoff B.