A fee can be added to Trades and Transfers. For trading fees, Koinly will add the value of the fee to the cost of the received coins. If the fee is in a cryptocurrency, Koinly will also realize any profit/loss on the fee itself. Refer to this blog post for more details on fee handling in Koinly.
Trading fees on tax reports
Note that you will not see trading fees in your tax reports since they are already included in the cost basis calculations to reduce your final taxable gains.
The Cost field on the tax reports includes any transactions that are explicitly tagged as Cost - this is not the same as trading fees!