How to customize the "User Invitation" email template?

According to User Invitation Email Templates it should be possible to customize the User Invitation email. However, in our IDN UI the User Invitation email template appears only in preview mode.

I’ve been searching for a candidate REST API to update this email template but couldn’t find a candidate. Can someone point me into the right direction ?

Thanks, Wim

If you select “Custom” from the Invitation Type dropdown, it allows you to edit the template.

Thanks Colin, I missed the dropdown