An On-Demand Flow invoked through the REST API is specifically designed for externally initiated, real-time automation in Marketing Cloud Next. Salesforce supports triggering this flow type by calling the Flow invocable-action REST endpoint. Input variables can be supplied with the request and then used by downstream flow actions.

That architecture is ideal for a one-time verification code. The external authentication service generates the code, calls the Marketing Cloud Next on-demand flow, passes the code and recipient information as input values, and the flow immediately uses those values in the Send Email Message action.

There is no reason to persist an ephemeral security value such as a one-time code into Data 360 merely so it can be read back by a Data Graph. Doing so adds ingestion and synchronization latency. Similarly, waiting for an Automation Event-Triggered Flow to detect streamed data introduces unnecessary infrastructure when the external system already knows exactly when the message must be sent.

The uploaded bank correctly keys C. Salesforce's On-Demand Flow release documentation explicitly states that REST API calls can trigger this flow type for externally initiated marketing interactions.

Study Guide Reference: Campaign Design, Flow Orchestration and Content → On-Demand Flows → REST API Invocation and Runtime Personalization.

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