The reason you're not getting results is because the Season you're requesting just started and there aren't any players with CSRs greater than or equal to 1800 yet. If you try the previous Season or a different Playlist in this Season, you should see results. You can cross-reference your results with the leaderboards here on Halo Waypoint (link
). I've updated the documentation for both the Halo 5 and Halo Wars 2 "Leaderboard - Player CSR" endpoints to clarify that only players with CSRs of 1800 are included in the response.
The leaderboard endpoint doesn't support a start or startAt parameter. It just returns up to the top 250 players (customizable with the count parameter).
Thanks for your interest in the API!