Reporting Players After Post-Game Lobby

I just recently had an experience where a player was very toxic outside of a league of legends match, behavior that I would normally report. The only problem was that since the match had already terminated I was past the post-game lobby where I could report. This player was urging people to kill themselves and calling them worthless saying "u need to kys" "end it" "just end it bro ur trash". I saved the chat log in a screenshot but I don't know how to proceed with reporting the player. Any help?