To top up your currency account, you can do so by transferring your funds from the main account to the desired currency account.
If you don't have funds in your main account yet, follow these steps:
Click 'Account' from the sidebar.
Click the 'Add Money' button
Choose the method to top up, either Bank Transfer or Card Top Up
Transfer the funds to your main account.
Once the funds are in your main account, follow these steps:
Click 'Payment'.
Click 'Between My Accounts'.
Select the desired currency account to which to transfer.
You will see the conversion rate from EUR to the currency of your currency account. There is no currency markup fee, meaning you can transfer from your EUR to currency account, and vice versa, for free!