Why do I get a 404 with Google's new vertex AI?

I'm using the example curl I get from:

https://console.cloud.google.com/vertex-ai/generative/language/my-prompts/create/text?authuser=2&project=my-app

The url it makes is:

https://us-central1-aiplatform.googleapis.com/v1/projects/my-app/locations/us-central1/publishers/google/chat-bison@001:predict

but 404 every time! (sending a POST, with auth, json, etc.)