Welcome, @MrFlapjack !
- Copy the URL of the extension. (For example, https://github.com/robo-technical-group/pxt-arcade-image-transform .)
- In your project, select the Extensions item in the toolbox.
- Instead of searching for an extension, paste the URL in the search box.
- The extension will appear in the search results. Now, you can add it like any other extension.