You must host the map on the web, for example on dropbox or on a web server in order to have a direct download URL.
For dropbox it will be necessary to replace?dl=0 by ?dl=1 at the end of the download url to make it direct which will give something like:
https://www.dropbox.com/s/random_characters/your_filemap?dl=1Then create or edit the server launch command line from the control panel > command line changer.
Check the +levelurl box to activate it and in the field in front of +levelurl put (copy / paste) the download url of your map.
So you will have:
+ levelurl https://www.dropbox.com/s/random_characters/your_filemap?dl=1

Save the command line and select it if it is not already done.
Finally edit the server.cfg file and put the name of the map in the server.level parameter which will give for example:

server.level "your map"

Check the name to be used with the map author but this must be indicated on the map page.
Save the modified file and restart the server.