To add a "Give" button to the Invitation email in Attendance, put your donation link in the Donation Website field in Event Setup > Event Details. That will automatically add a Give button (as shown above) with a link to your donation site to your invitation emails. When a guest clicks on the Give button in the invitation email, it brings them out of FundEasy and to the giving site that you entered in that field. This action does not count the guest a declined registration in FundEasy.
If your invitation email has a "Give" button and you would like to remove it, remove the URL from the Donation Website field in Event Details. There isn't a way to remove it on the template itself.
This article has more information about Event Details.
To add a Give button or link to other emails, use the edit link in the right corner of the template preview in Event Setup > Web & Email Templates and add the image and link to the text editor.