It won't save it online, you have to copy it and then paste it in Word or Wordpad or Notepad, or any other program that you have for writing documents.
Open one of those and paste the copied message and save it there until you are ready to finish it.

Another way that might be simpler is to make your partial post and add at the bottom that you will finish it later on. Then make the rest of the post as a new message in the same thread if it is beyond the editing time frame.