If you need to add a new Purchase Order to an existing Placement, you'll need to navigate to the Placement record and click on the Finance tab. Then click Add Existing Purchase Order :
Click on + New Record
Click Purchase Orders
Click OK as you are being redirected to create a New Purchase Order
Complete the New Purchase Order as required by your organization. Click Save & Close and you'll be taken back to the Placement record.
Once again click on Add Existing Purchase Order
You will see your newly created Purchase Order at the top of your Recent records list. Click it and then click Add at the bottom.
The Purchase Order is now associated to the Placement record and the Client record. Purchase Orders can be used on multiple placements if necessary. Don't forget to click Save at the top.