It is not possible to install custom plugins on our Standard or Business hosting plans. All plugins available on those plans are pre-installed for you.
For pushing data from Discourse to Segment, have a look at this theme component: GitHub - scossar/discourse-segment-theme-component. Details about how to install theme components are here: Installing a theme or theme component.
Let me know if you have any questions about the theme component, or if you run into any issues with using it.