Supporters posts are written in the same editor as Notes. It opens in the Formatted view; Markdown, beside it, replaces Edit as Markdown in the post's More menu. The toolbar has the same commands as before, and a picture can be pasted into the text to upload it. A video that is not finished, or that was deleted from Screen Recorder, shows in the editor as a line saying so instead of disappearing from the text. ⌘K opens Search in a post as everywhere else; use Link in the toolbar to add a link.
Supporters posts use the Notes editor
Posts are written in the same editor as Notes, with Formatted and Markdown views of the same text.