Well, I use the rutorrent GUI to create my torrent, so it's graphical 
But according to the documentation of mktorrent, I found this:
--announce=<url>[,<url>]
So it seem that you need to had the argument --announce (or just -a) followed by each url and separated by a comma.
As for the URL themselves, you just need to search some public tracker to add. I found an auto-updated list on github: https://github.com/ngosang/trackerslist