To use the "Link a Device" feature on https://plex.tv/link code you will need to open the Plex app on the device that you want to link and then enter the code provided by the Plex website when prompted. This will link the device to your Plex account and allow you to access your media library from that device. If you are having trouble with the linking process, you may want to check the documentation provided by Plex or reach out to their support team for further assistance.