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Associate a Purchase Order to a Client record. Find through: Clients > Client record > Related tab > Purchase Orders
 
 
 
 
You'll see existing Purchase Orders or none at all if this is the first one. Click +New Purchase Order

Complete the fields required marked by a red asterisk (*), then click Save & Close to return to the Purchase Orders tab of the Client. 
 

To edit a placement order, click the tick-box to bring up the top ribbon bar. From here, Edit, Delete, Assign, etc.