The 'Are you sales tax registered?' part of the Portal is for UK based clients only, who are VAT registered. It's good to fill this out even if you're non UK so we're up to speed on your tax status however we won't need anything further from you.
As a non UK based AWAL member, you may have withholding tax deducted from your account balance. If this happens it will be shown on your statement. Please see our article about withholding tax for more information.