It is at the end of the filing process, where you choose how you want your refund - direct deposit, apply to next year, check, etc.
You can designate up to $5,000 of your federal refund toward up to three Series I bonds, which will be mailed to the address on the your return. You'll see the option to purchase bonds when you get to How would you like to receive your federal refund? in the File section.
When you get to that screen, select Split my refund between multiple accounts or U.S. savings bonds. Select Continue and follow the instructions.
From this TurboTax FAQ
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