In Power Automate, create a new Instant cloud flow and add the 'When an HTTP request is received' trigger; define a JSON Schema for the expected request body to enable dynamic content parsing in subsequent actions.
Save the flow — Power Automate generates and displays a unique HTTPS POST URL; copy this URL as it contains an embedded SAS token and is only shown after save.
Send a POST request from your external system to the generated URL with Content-Type: application/json and a request body matching the schema you defined.
Configure the trigger's 'Who can trigger the flow' setting: set it to 'Anyone in my tenant' (requires a valid Entra ID bearer token) or leave it as 'Anyone' for unauthenticated access, depending on your security requirements.
Map values from the HTTP trigger body to subsequent flow actions using the triggerBody() or @{triggerOutputs()['body']['<fieldName>']} dynamic expressions.
Return a custom HTTP response to the caller using the 'Response' action before any async actions, as the caller waits synchronously for up to 120 seconds; for longer-running flows, respond immediately with 202 Accepted and a correlation ID.
Known gotchas
The generated trigger URL contains a SAS signature that is tied to the flow — if the flow is deleted and recreated, the URL changes; store the URL in a configuration system rather than hardcoding it in callers.
The HTTP request trigger does not validate the Authorization header by default when set to 'Anyone'; anyone with the URL can trigger the flow — use the tenant-scoped option or add a secret header check in the flow for production scenarios.
The synchronous response window is 120 seconds; if flow actions take longer, the HTTP caller receives a 504 even though the flow continues running asynchronously — design long-running flows to return 202 immediately.
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