Customizing Stream Highlight output (to file or chat response) uses [help-commands: Custom Command] syntax, with the following parameters:

Examples:

$(timestamp) $(chan) [$(uptime)] $if(chatuser,[$(chatuser)] )$(rawcomment)$if(addedmarker, (Created Stream Marker\))
Close to what gets written to file by default.
Added stream highlight [$(uptime)] $(comment) $if(chatuser,\(This is a mod-only command\))
Compared to default chat response this omits some information, but adds a note about who can run the chat command (but only if actually ran from chat command, note escaping the parentheses as well).

Note that for customizing the chat response, there may be some error messages that can't be customized.