Broken/NS102:Mainurl
See all Organizations and their Main url's.
Do I just put any old url?
No. The main url is the overarching url in which all resources are housed. For example, the main url for the Open Society Institute is: http://www.soros.org/, even if the site you stumbled across initially was http://www.soros.org/resources. In this case, you can put the latter into the Resource url field, since this url takes you directly to some of the resources on the site.