Go to your Poshmark, Depop (or any other supported platform) shop. A little icon should be displayed in the top left corner of any listing. Clicking on it opens a new tab where a draft of the listing is created on your target platforms. You should review this draft carefully, and if you are happy with it, submit it.
What to do if the icon not showing up?
Doing a hard refresh (Ctrl+Shift+R on Windows, Cmd+Shift+R on Mac) might work. Restarting the browser also might help. If neither of these help, try this experimental webpage: https://www.crosslistmagic.com/crosslist