As title says, i tought that it would have been automatically assigned, but i guess not now… Can someone help me out? 
Since this is in the OM section, I think you mean the mossgolem? In order to qualify for the prize you need to finish within so many people. If you haven’t won a prize usually 15-20 minutes will qualfy you for the prize, if yor haven’t won anything you have 20-30-45 minutes (sometimes an hour if people arnt as interested in the OM) to qualify depending on the size of the dungeon.
So if you qualify, then yes it is automatic when you finish otherwise, it states “All pries have been claimed”
If it doesnt then ur experiencing a bug