Thank you for your quick response George.
I tried your (StackOverflow) suggestion, I copied the email template file to my (child)theme (with the right paths). I edited some things, which works great. But I think/read JavaScript is not supported in e-mails.
If I could redirect customers to a page within my website where they can print their tickets, it would be a good solution as well, but that’s not possible is it? Maybe i nice feature for future updates.
Thank you for your help anyway,
Peter