In shopify.extension.toml, add a targeting block with target = 'purchase.checkout.contact.render-after' under [[extensions.targeting]] for a static placement after the contact information form
Use useApplyAttributeChange() from @shopify/ui-extensions-react/checkout to write custom key-value data to the order attributes when the customer fills in a custom field
Render a TextField component bound to a local state variable; on change call applyAttributeChange({ key: 'custom_field', value: inputValue, type: 'updateAttribute' })
Use useBuyerJourneyIntercept() to validate the field before checkout proceeds — return { behavior: 'block', reason: 'Field required', perform: ... } if the field is empty
Test the extension locally with shopify app dev and visit the checkout URL in a test store; extension console output appears in the terminal
Read order attributes after purchase via the Shopify Admin API on the order object's note_attributes field or as metafields depending on your data model
Known gotchas
Static targets like purchase.checkout.contact.render-after are hidden when the checkout section they relate to is not rendered (e.g., when buy-now links skip the contact step); block targets are more reliable for always-visible content
useApplyAttributeChange writes to note_attributes on the order, not metafields — the data is not indexed and cannot be queried via GraphQL metafield queries without a separate metafield write after order creation
useBuyerJourneyIntercept blocks checkout progression only if the extension is assigned to steps that include the validation point; it cannot block the final payment submission step
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