Once a post has been created, you can always save the post and come back to it later. Just click 'save draft'.
You can only save the subject and the message, any other element, such as stickies, emojis, hyperlinks, images etc, will be removed.
To return to the draft, just click 'new topic' in the thread you saved the draft in and you will then get an option to 'load the draft'. Click the subject you created, and continue with your draft.