Hi CharlyTalavera,
Currently, Shopify Flow does not support setting default values for custom fields in a flow action. The merchant who uses your action must set up this field manually through the Shopify Flow UI.
Hope this helps,
Topic summary
Goal: Add two inputs to a Shopify Flow action—Order.id (required) and Fulfillment.id (optional)—and auto-populate Fulfillment.id from the trigger if available.
Attempt: Configure a default in shopify.extension.toml by setting a single_line_text_field (key: fulfillment_id) with value “{{fulfillment.id}}” so the action receives it by default. The code snippet is central to the request.
Outcome/Current status: Shopify Flow does not support default values for custom fields in flow actions. There is no built-in way to prefill a custom field from trigger data via the extension configuration.
Impact: Merchants must manually set the optional Fulfillment ID in the Shopify Flow UI when using the action. The desired auto-population behavior isn’t currently achievable.
Resolution: Confirmed limitation; no workaround provided. Discussion concluded.
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