To use a Promo or Referral Code, select the blue 'Promo or Referral Code?' button during your payout request:
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Then you'll need to enter the promo code into the Promo Code field and select 'Apply' for it to be applied to your payout:
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After you've applied the Promo or Referral Code, you just need to accept the Terms and then click on 'Transfer Now'.