openapi.yaml, not narrative guides.
Authenticate with Authorization: Bearer sk_live_… or sk_test_…. See Authentication and Environments in Guides.
The playground base URL comes from OpenAPI servers (and api.mdx.server in docs.json). Use the server dropdown to switch between Local (ngrok) and Production. After changing those files, restart mintlify dev and hard-refresh the browser.
For free ngrok, add request header ngrok-skip-browser-warning: true in the playground if the response is an HTML interstitial instead of JSON.