Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
Nextcloud version (eg, 20.0.5): 28.0.0
Operating system and version (eg, Ubuntu 20.04): Ubuntu 22.04 Server
Apache or nginx version (eg, Apache 2.4.25): Nginx
PHP version (eg, 7.4): 8.1
The issue you are facing:
Is possible and if so how to add a new default category to the calendar event categories list.
This would ensure that that any of the users would already have that category present when creating an event.