You can plan workforce for WBS under "Resource Planning" tab -> "Effort Resource Planning" tab.
WBS Project Source Information
In the "Organizational Role" field, you can select a " Management Organization Unit" from the window and the system will assign the "Position Holder" to the "Personnel Cards / Business Partner" field. If you want, you can edit this position holder.
The Labor Cost Definitions made for this "Organizational Role" will then be used in the cost calculation for the project WBS and therefore the "Project" to which the WBS belongs.
WBS Project Resource Cost Planning Information
You can select a "Cost Element" from the Service Products window.
For this, you can select a Unit and type "Amount".