You don't.
Riot doesn't accept disciplinary reports via the support ticket system and pre/post game chat are, for whatever reason, not monitored. The old Riot excuse for this was a technical limitation in the client, IIRC. I have no idea if the new client is capable of this, but I'm willing to bet not as I seem to recall Phreak mentioning on stream not too long ago that your best option was to submit a report the normal way and make sure to write in the box that the offending chat happened in pre/post game. It doesn't actually do anything, but his reasoning for this was that if enough people report the player in question for pre/post game chat, that the system would eventually trigger a manual review.