The Simply Wall Street platform does not account for brokerage costs or fees in your Transactions Portfolio.
However, if you would like to manually add the cost of brokerage into your trades to better calculate accurate returns in the Detailed Returns report, what you can do is include the cost of your brokerage per trade in the price of the Buy or Sell of your transaction.
How to include transaction costs
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