How do I find the API endpoint of a lambda function?

I don't really understand the above answer (maybe it's outdated?).

The absolute easiest way:

1. Choose "API Gateway" under "Services" in AWS.
2. Click on your API.
3. Click on "Stages".
4. Choose the stage you want to use
5. Now you can see the entire URL very visible inside a blue box on the top with the heading "Invoke URL"

Your API Gateway endpoint URL doesn't get exposed via an API call. However, since the URL of the API follows a certain structure, you could get all the necessary pieces and create the URI within your code.

https://API-ID.execute-api.REGION.amazonaws.com/STAGE

You could use apigateway:rest-apis to get your API-ID and restapi:stages to get the stage corresponding identifier.