Custom Registration Handle URL - Can't find it

We have an external event where we will not handle the registration. We want to direct our users to external site using the Custom Registration Handle URL as per doc
eventbookingdoc.joomservices.com/basic-s...events/basic-concept .
The problem is I can't find it on the Event detail page?
I have checked that no plugins need to be enabled or anything in the Config options.
Can someone direct me to where I can set the url for the event?
Thanks in advance

Hello

The URL won't be displayed on event details page. If the URL is entered and registration for that event is enabled, there will be Register button displayed. Click on that button redirect users to external website for registration

Tuan

Hi Tuan,
Apologies, I did not explain very well, When I said 'Events Details Page' I mean the page where the administrator enters the URL. I can't see where to enter the URL
See screenshot of Event Page in backend.