Thank you for reaching out here.
This customization is for vintage Shopify themes ( such as Boundless, Brooklyn, Debut, Express, Minimal, Narrative, Supply, Venture ) and doesn't apply to Online Store 2.0 themes and the Simple theme.
So you should try with above-mentioned themes to achieve this feature or hire an expert to do it for you.
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After rechecking on my Dawn Online Store 3.0 and 2.0, I found this feature in Subtotal Section.
You could go to Theme-> Customize. In the dropdown at the top center, select Cart and you'll want to click on the Subtotal section on the sidebar menu and you'll see the option Enable cart notes as picture below:
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