TaskList Uri

I use the Api to extract the project data. I try to form my Uri, and query the tasklists for a given project. I have difficulty adding the flag parameter. Please advise how to add the flag parameter. The response returned is 400 bad request.

My attempt:
https://projectsapi.zoho.com/restapi/portal/ourrepository/projects/123456789123456789/tasklists/&flag=external/?authtoken=someguidtoken