Spread is the difference between the bid and ask price. Forex brokers don’t charge their clients for a transaction, instead they take the difference between the price you bought the currency and the price sold. It seems like you’re not losing any portion of your capital when the trade is going your way but in a nominal way and depending on the lot size, the broker already has their cut from that transaction.
Forex brokers provide their clients a dealing desk giving them the capability to execute orders in real time. When placing a single mini lot buy order on the EUR/USD currency pair which normally has a 3 pip spread, you’re already down $3 dollars since the pip value for that pair is $1. The amount increases depending on the lot size.