The Simply Wall St platform does not have a dedicated field for brokerage costs or other fees in the Portfolio tool.
However, if you would like to manually add the cost of brokerage into your trades to better calculate performance, what you can do is include the cost of your brokerage fees in the total cost of your investment - also known as the average price of your holdings.
How to get the average cost
To do this, simply get the total cost of transaction from your broker's account then divide it to the number of shares transacted the result should be your adjusted price to input in the system.
If you bought 2 shares of NasdaqGS:AAPL at $125 with $50 as brokerage (and other costs), you compute the adjusted price as:
Adjusted Price = Total Transaction Cost / Total Shares,
where, Total Transaction Cost = (Total Shares X Price) + Add on cost
Adjusted Price = [(2 X 125) + 50] / 2 = $150
Thus, instead of using the $125 as your price, you are going to use the adjusted price at $150