There is currently no way to set custom claims for a user in the Firebase console. If you think this would be a useful addition, file a feature request for it.
In the meantime the easiest way to add a custom claim is to do so from the a terminal window/command prompt using a small Node.js script like this:
admin.auth().setCustomUserClaims(uid, {admin: true})
Also see:
- How to create Firebase Authentication claims?
