The default currency account used for investing depends on the base currency of the instrument. You should fund your currency account(s) before you place an order. You are required to have sufficient funds in your chosen currency account else, you would be charged for financing.
E.g. If you choose to invest in a US instrument and have a USD currency account, the USD currency account will be the default account used when placing an order.
For more information on executing a trade on another currency account, refer to ‘Trading Related’, Question 10.