As far as I know, you should be able to use Gitpod. If you pin the workspace it shouldn’t get deleted and I think the live URL persists (remains the same). So that might be an option.
Side rant.
The requirement of a live URL is honestly a bit questionable. There are plenty of projects that have passed the tests and were submitted as a solution that is not functional anymore.
Pretty much any project that uses MongoDB Atlas will have the cluster paused after some amount of time and as a result, the project is not functional and can’t be easily checked. Depending on where and how the project is hosted checking it might be impossible.
It would be just as easy, if not easier if the project submission was a GitHub repo. At least then you would have guaranteed access to the source code.