I made a .bat containing this:
blocklandlauncher.exe -dedicated -gamemode speedkart
Inside the server's prefs file, I have this:
$Pref::Server::UseRTB = "1";
Finally, I changed the speedkart gamemode. Inside its gamemode.txt i added this:
ADDON System_ReturnToBlockland
I then made speedkart read only so the launcher would not overwrite it.