With in a work order on the Time and Materials tab there is a line item called Purchase Orders
In the Description field you would enter either the name of the supplier where the materials were purchased with the $ spent and click the check mard to save. (The above example has the auto generate PO# turned on).
The auto Generate PO#'s is turned on on the Features page in settings, the starting number can be set on the Tenant page in Settings.
Most companies choose to not have the auto generate PO's turned on and use the work order number or til receipt number as the PO number.
Once this work order has been completed and an invoice created for the work completed, any information that was entered in the Purchase Order section is transferred to the Materials section of the Invoice.
If the supplier name was used it might be useful to break this line item out into the individual items to keep track of item pricing going forward.