Something is wrong with the code on that page. It says:
-- Derive chat message properties
local properties = TextChatService.ChatWindowConfiguration:DeriveNewMessageProperties()
This will return ChatWindowMessageProperties, while the whole page implies it would return TextChatMessageProperties. Later the code uses properties.TextColor3, which is not listed in properties below, because it isn’t a property of TextChatMessageProperties, but of ChatWindowMessageProperties
The page of ChatWindowMessageProperties is empty, and it seems like that piece of code should be put there instead.
At the end, the callback returns those properties. It is not necessary, because the callback expects TextChatMP, and it does nothing with ChatWindowMP.
It seems like those two things got mixed when writing those articles (which might be a sign that this system is (maybe unnecessarily) complicated)
By the way, is it intended that I can’t create ChatWindowMessageProperties with Instance.new, while I can create TextChatMessageProperties?
Page URL: https://create.roblox.com/docs/reference/engine/classes/TextChatMessageProperties