First of all, you need to select the keycloak realm.
On the left menu, click on the user tab and click on the Add User button
Create your new user with the form that appears, validate by clicking Create.
Once you're done, click on that freshly created user and in the Role mapping tab, click Assign role.
On the top left dropdown of the popup that opens, select Filter by realm roles, and assign the admin/role_admin to the user.