When opening a Forex trade, a certain amount is deducted from the trader's balance. This amount depends on several criteria: trade amount, multiplier, asset specification, etc. The current commission is shown along with the rest of the information about the trade. However, the final payment may sometimes differ slightly due to the market situation.
Information about the minimum commission rate for opening a trade and other conditions can be found in the "Trading Conditions" tab of the "Assets" menu. You can access it through the "Help" section.