How can I add little tick marks in the progress bar to a Kindle ebook? Tick marks indicate where chapters are so we can tell how close you are to the next chapter.
If you mean the little dots that appear below the book name in the Kindle device/app library, you can't. Amazon determines how many dots to show, and they apparently do it based mostly on file size. There's no way to tell Amazon how many dots to use or that a particular dot corresponds to a particular portion of the book.
If you mean adding ticks to the progress bar on the bottom of the reading screen, that isn't possible. Authors don't have any access to those features of the device or apps.