You can already call an external API from Briefer and use its response within the notebook.
You could also upload any types of files and read them from disk - or even query those files using DuckDB if you want.
Edit: Maybe I'm misunderstanding the UI and this is something you would do directly through Python?
To pull data from the API you can use the requests package which is already within your Python environment.
If you have sensitive API keys you can add them to your environment variables list and then read them with Python too so they’re not apparent in the code.