Normally, you cannot. The broker will not allow you to lose more than the available funds on your
trading account. It will simply close your losing position when the resulting
account balance becomes too close to zero. The loss that is bigger than the
trader's deposit is a direct loss of the Forex broker. It is in the brokers'
interest to prevent such losses. To secure themselves brokers implement a
Stop-Out level (usually about 20%), which means that the most losing position
will be closed once (equity / used margin) × 100% becomes equal to or less
than this level.
In rare cases, a slippage or significant price gap may put the trader's balance into negative territory. However, brokers rarely pursue traders to refund negative account balances.