I don't see a forum for Carnival Horizon. How can one get started?
June 25 sailing
At the bottom of this page, in the center column. click on "Contact Us" and describe what you want.
The staff here are quick to fix any problems.
(Although, I think Simon (the resident tech) is currently on vacation, but I think the rest of the staff are still there).
EDIT - I just checked the roll call, and it seems to be there. If you were talking about a separate forum, then I think this site does not offer that -- only through the search function. If you were talking about the roll call feature, it is at the top menu bar, under "More", "Roll Calls". Enter the line (Carnival), the ship (Horizon) and the date (June 2018) and you should then find it from there. If not, try refreshing your browser (deleting the cache).
Hope you have a great cruise, and welcome to the forum!
BDRebel gave you the correct advice.