Asked on 21 Apr 2019
Yes, and this (obviously not so visible fees for conversion) is exactly the reason why, when your primary currency is SGD, and You want to buy USD (or other foreign currency) based equity, you should (even with a Singapore based broker!) switch the currency via a much cheaper company. Cheapest one as far as I know currently Transferwise (only ca. 0.30 % !).
Order USD on Transferwise for Your SGD sum and tell them to which account (i.e. Your USD nominated brokerage account) to send the USD, pay Transferwise by one of Your SGD nominated accounts, wait 2-3 days, USD arrive at Your brokerage account, ready for action ...
For me I transact in USD on the brokerage platform only, and use DBS mca to do the fx conversion. So I only convert to USD if I'm looking to send a wire to my account
I could be wrong but I believe that the currency conversion is done through the respective broker's exchange rate.
For Saxo Capital Markets, "Currency conversions of trading costs as well as profits and losses from trading activities are executed at the mid FX Spot rate when you close the position, plus/minus 0.5 % for settlement of actual payments to or from the trading account are included, for example, buying/selling cash Stocks, paying/receiving options premium etc.".