The contents and folder hierarchy of the game server to be downloaded via HTTP need to copied to an HTTP server starting from the root of the game(i.e., copy /cstrike/maps/... or /tf2/maps/... from the game server to /cstrike/maps/... or /tf2/maps/... on the HTTP server).
Set sv_svdownloadurl to the appropriate value and ensure sv_allowdownload and sv_allowupload are both set to '1'. The recommended location for these cvars is the server.cfg file. Be sure to replace DOMAIN with the appropriate domain or IP address in the example below.
sv_downloadurl "http://DOMAIN/cstrike/"
sv_allowdownload 1
sv_allowupload 1