When adding a song that is in a co-publishing agreement, you should follow the same steps as you would if you were registering a song that you own 100% of your publishing share.
The only difference is that you will have to add yourself as a writer twice - once as a client writer to represent the publishing share you own, and once as an outside writer to represent the publishing share that you do not control.
So for example, lets say I, Asia, wrote a song called "My Song" and I only owned 50% while Universal owned the other 50%. I would register this work into my account as follows:
Asia - 50% (client writer)
Asia UNIVERSAL - 50% (outside writer). I always recommend adding the publisher in the name of the outside writer so it is clear that this song is in a co-publishing deal.