TaskList Uri

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