Next-gen projects are independent, it means that it's not possible to add system custom fields to next-gen projects.
For example, the workaround provided on https://jira.atlassian.com/browse/JRACLOUD-31592 will not work for this projects because we can't add the "Free Text Field" and the Text field provided on next-gen project has the limit of 255 characters.

Suggestion:
Ability to use the same custom fields as any other project or add more options on next-gen projects.