Both Operations and Inventory processes have an ‘Add Cost’ Button when being viewed.
Here we have a Packaging Run. This Operation is unique in that it can have both a Operation Cost and a Inventory Cost. When the Operation/Inventory process has more than a single choice to apply a cost to (such as this, Multi Additions, Transfers, etc.) a dialog will appear.
Click on the drop down to see the possible choices to apply the ad-hoc cost.
Once you select a target, you will see the Add Cost Form.
Fill in the form as you see fit. Then check back on the cost in the Cost Admin Console. You will see the cost is linked directly to the selected operation.