Hey Holly18,
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To add a button for visitors to register interest in a specific size, you could either customize your theme or use Shopify’s built-in forms. Create a size dropdown next to the purchase form, where customers can select their preferred size. For ease, you could have an option to notify them via email when the size is restocked. Alternatively, you could add a simple form that captures just the size they want, without needing an email, and review the data manually.