Once you've completed the Rent-to-Own program for Serato Sample, you can find your serial key here: https://splice.com/plugins/your-plugins.

Open your Serato Sample plugin in your DAW and it will prompt you to enter that serial number.

Create a Serato.com or Sign-in to your existing Serato.com account

Navigate to the Products page in your Serato.com account

Enter your voucher code via the ADD A NEW PRODUCT LICENSE section (including dashes)
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Then download, install and activate Serato Sample on your computer using the Activation steps from the Serato Sample activation & deactivation article.