Under the “Files” tree is a “Dependencies” section. You can add your dependencies there. If you just want the latest version you can just use the package name (ex: ngx-bootstrap) or you can target a specific version (ex: [email protected]).
Update 9/5/2022 NextJS
It looks like dependencies are not auto installed for NextJS projects. You do have access to a terminal under the code editor though. If you set focus to this terminal you can Ctrl+C to stop the dev server. You then can install you dependencies from this terminal.
npm install --save <package-name>
Then just restart the dev server with…
npx next dev.
Hopefully this helps for NextJS users.