the ReNoise songs page is not offering hosting for your songs: you already have to have your songs online before submitting them to the songs page.
Once you have put your song somewhere in the cyberspace, you have to write the song URL in the proper field.
NOTE: there are 2 “song URL” fields:
one is to be used if you want to release the RNS version of the song, the other if you want to release an MP3/OGG version.
You can obviously use both fields.