Hi, Annie, when I use a PC and I am in the piece I want to publish, there are several options at the top right of the screen — the first one toward the left is “Share”, the next one is “Publish”. If you click the down arrow next to Publish, you come to a screen that allows you to add the tags.

Here is a link to the explanation in Medium:

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store