Please click on the red gear button, which appears when you hover your mouse cursor over the nickname in chat.
Select "Send CURRENCY to" option. This will automatically fill the command.
All that's left to do now is to enter the amount you wish to send and confirm.
Note: All BUX operations need to be confirmed either via an e-mail code, either via the 2FA code on your phone.
Note: "Currency" will be BUX, USD, EUR, etc, depending on your personal display options.