If you're giving a custom warning, you need to write it manually (logically). But if you use a pre-created warning, it can pull a default conversation:
There is the option of starting a conversation with the member being warned. If a pre-defined warning has been selected and the Conversation Settings have been completed for the warning in the admin control panel, then the title and content fields will be pre-filled accordingly. This can be edited when issuing the warning if required.