BetaTesting rewards are redeemed through Tremendous, which offers over 2,000 redemption options in 200+ countries. The options available to you are based on our validated location. The Visa International Gift Card option can be used at most countries around the world.
You do not sign up for Tremendous yourself, and you cannot. Only companies that send rewards are able to create Tremendous accounts. Instead, you request a redemption from BetaTesting, and then you receive an email from Tremendous with a direct link to redeem your reward.
For the full details, see Redeeming Rewards with Tremendous and How can I access my Tremendous balance?