When the API_TYPE argument is set to "aws-forecast", API_Request in-database function works with Amazon Forecast. The syntax is:
SELECT * FROM tapidb.API_Request( ON { table | view | (query) } [ AS InputTable ] USING AUTHORIZATION('{"AccessId": "AWS_access_id", "AccessKey" :"AWS_Access_Key", "Session_Token": "AWS_Session_Token", "Region" : "aws_region"}') ENDPOINT('forecast-endpoint') API_TYPE('aws-forecast') ) AS alias;