If you have built landing pages with the forms in marketo which the visitor will have to go through first to end up on your website you can use the URL builder to help you create the parameter value
Once you're in there it will give the hidden url parameter to fill in with whatever you like and make it as long as you like. It will then give the complete url to use. Avoid using any special characters like !"%$&, and to space out the words you should use _ or -. An example would be: marketo_community_2015-03.
I really hope this helps