Problem
You have a service that needs the firstname of a user and to make sure, that every account has that field filled out, you want to make it mandatory. Per default, the firstname field is optional.
Solution
Create a new ucr variable with the udm object firstname:
ucr set directory/manager/web/modules/users/user/properties/firstname/required='true'
Then restart the UMC to apply the change:
systemctl restart univention-management-console-server.service