Can you provide some context as to why you need it via a API call rather than with the UniversalPackage task?
If you do need to orchestrate the download in more of a script environment (powershell/bash) - I'd suggest looking at the AZ DEVOPS CLI.
- Az DevOps CLI - Download package
- Az DevOps CLI - Download file
The DevOps CLI provides a really nice wrapper for the popular API's.