Trigger a Shopify Flow from an external event using a custom connector

Steps

1. Create a Shopify app and add a Flow extension; define the trigger in shopify.extension.toml under [extensions] with type = 'flow_trigger', specifying the trigger title and any payload fields your external event will send.
2. Deploy the extension with Shopify CLI so the trigger appears in Flow for merchants to use in automations; no write_flow access scope is required — the trigger is registered via the extension manifest, not an API scope.
3. If your trigger involves customer data, include the read_customers scope in your app's access scopes so Flow can resolve customer references when the trigger fires.
4. From your external system, send a POST request to the Flow trigger endpoint provided by Shopify for your registered extension, including the shop domain in the header and a signed payload using your app's client secret.
5. Shopify validates the request and fires any merchant workflows that use your trigger; inspect the response status (200 indicates accepted) and log errors for retry.
6. Test end-to-end by creating a simple Flow automation in a development store that uses your trigger and verifies an action (e.g., add a tag) fires when you POST the event.