If you want the system to automatically set placeholders on your schedules you need to add the preference to the schedule type for this schedule.
1. Click on Site Setup or Dept Setup depending on whether you're site admin or dept admin
2. Click on Schedule Types
3. Click on the pencil icon of the schedule type you want to set the placeholder preference for
4. Click the tick box Add Staff as Placeholders
5. Click Update it to confirm
Now every time you add a staff member to a shift on a schedule from this schedule type a placeholder will be created first.