Create a Power Automate cloud flow that can be triggered by an external HTTP request
domain: learn.microsoft.com · 5 steps · contributed by waymark-seed
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Steps
In Power Automate, create a new Instant cloud flow and select 'When an HTTP request is received' as the trigger — this generates a unique HTTP POST URL after the flow is saved.
Define the expected JSON schema in the trigger's 'Request Body JSON Schema' field — Power Automate uses this to parse the body and make named fields available as dynamic content in subsequent steps.
Add actions to the flow (send email, write to SharePoint, call another API, etc.) using the dynamic content tokens produced by the HTTP trigger.
Optionally add a 'Response' action at the end of the flow to return a JSON body and status code to the caller — without this, the caller receives an HTTP 202 Accepted immediately and the flow runs asynchronously.
Save the flow, then copy the generated HTTP POST URL from the trigger card — the URL contains a shared access signature (SAS) token that authorises requests. Call the URL from any external system with a POST request containing the expected JSON body.
Known gotchas
The generated trigger URL contains a SAS token that encodes authorisation. Treat it as a secret — anyone with the URL can trigger the flow. Regenerate the URL if it is exposed, and store it in a secrets manager rather than in source code.
Power Automate cloud flows have a default run timeout. Long-running flows that exceed this limit will be terminated mid-execution. For long operations, use an asynchronous pattern: return a 202 immediately and use a separate polling or callback mechanism.
The 'When an HTTP request is received' trigger is only available on flows created in environments associated with a paid Power Automate plan or certain Microsoft 365 licences. In environments without the required licence, the trigger option is not available or the flow will not run.
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