If it is for a single user, you can go the user’s preferences directly and edit the notifications for them:
If you need this for a specific group of users, you could create a Group (read more here: Configure custom group Access and Visibility) with default notifications in certain category or tags. For example if I want the members of the group “Test” to always receive notifications for Topics in the “Tests” category:
And then send an invitation (read more about it here: How to send, configure, and accept Discourse invitation emails) to those users, adding them to the group, like so:
Once the users are part of the group, the notifications settings will be updated in their Preferences page to include the group’s notification defaults: Groups can set category and tag notification levels
You could also set the default categories watching first post, default categories trackingand default categories watchingsite settings. But these would apply to all users not just a specific group.