Authenticate to the Marketing Cloud REST API using the OAuth 2.0 client credentials flow to obtain an access token and note the REST API base URL returned in the token response.
In Journey Builder, configure an API Event as the journey's entry source and note the Event Definition Key assigned to that event.
Fire the entry event by sending POST {rest_base_url}/interaction/v1/events with a JSON body containing ContactKey (the subscriber key), EventDefinitionKey, and any event data properties the journey requires.
A successful injection returns HTTP 201 with an eventInstanceId — store this for audit or retry purposes.
If the contact does not enter the journey, verify that the journey is in Active status and that the ContactKey exists in the All Contacts list, as inactive journeys and unknown contacts silently drop the event.
Known gotchas
The journey must be in Active status before contacts can be injected — posting to a Draft or Paused journey returns a 400 error.
The Event Definition Key is case-sensitive and must exactly match the key shown in Journey Builder; a mismatch produces a 404 or a silent drop.
OAuth access tokens for Marketing Cloud REST API have a finite lifetime — implement token refresh logic and do not hardcode tokens.
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp