If you want to add COGS/COS to your model you can do so by following the steps below:
Navigate to the Financial Model Builder page and the Expense tab.
Create or edit an expense.
Click the "Link revenue" dropdown menu and select the revenue stream you'd like to apply the expense as COGS to.
The expense will now automatically be factored in as COGS/COS in your model.
If you can't see the "Linked Revenue" column
You can add the columns you like by selecting which columns are displayed via the button just above the model builder.
See also what is COGS/COS.