While the pre- and postgame lobby chats are not included with the reform card, they are recorded and can be pulled and reviewed at any time by Riot. The easiest way to report a player based on these chats is to open the report field at the post-game lobby, and down in the additional comments box, type in something referencing them. "Toxic in pregame and postgame chat" "please review postgame lobby" "hate speech in champ select" Anything like that, just something that tells the system about them. I'm not sure if it triggers the IFS to review them further, or if it triggers one of the Riot staff to review them, perhaps one of the Arbiters will be able to elucidate that a bit.
The more time consuming way, which is not preferred, is taking screenshots of the chat in the lobbies. Continue to submit the report in the postgame lobby, just as you normally would, but afterwards you can then submit a ticket to Player Support and upload the screenshots in the ticket. They will be reviewed and added into the file for the account that was reported, and any action that needs to be taken will be taken. However, due to player privacy, they will not be able to tell you whether they hand out a punishment or not.