If you are a mod
You can generate an access token for yourself
Then use the chat topic of chat_moderator_actions.yourID.channelID
Then when someone in channelID starts a raid you will get a message along the lines of
{"type":"MESSAGE","data":{"topic":"chat_moderator_actions.USERID.CHANNELID","message":"{\"type\":\"moderation_action\",\"data\":{\"type\":\"chat_channel_moderation\",\"moderation_action\":\"raid\",\"args\":[\"TARGET\"],\"created_by\":\"NAME\",\"created_by_user_id\":\"NAMEUID\",\"msg_id\":\"\",\"target_user_id\":\"\",\"target_user_login\":\"\",\"from_automod\":false}}"}}
*example message is an example and may not match what pubsub currently sends