Don't fill in anything just click payment to faucet box above the text field.
Logged by email, and i not fill address, press Withdraw Balance to Faucetbox and see error "Invalid 'to' address."
I filled my btc address before first claim (where it says "Enter your Faucetbox or XAPO Mail here and start to claim!") and got paid to faucetbox by just clicking pay to faucetbox.
I can't test how it works because my balance is 0 now and I have to wait 300 minutes before I can claim again.
But I think most of people will fill their preferred payout address already at first claim so it will work for most people same way it worked for me.