This was the "exclusive" content the cryptic link had promised. But it wasn't a loot box or a skin pack. It was a series of challenges.
However, the string suggests a different context: a fork, mirror, or custom build hosted on GitHub Pages (all *.github.io domains are static sites hosted by GitHub). httpsmathsframegithubio exclusive
If you're having trouble accessing the content or need more information, consider reaching out to the support team for Maths Frame. They should be able to provide guidance on eligibility and access. This was the "exclusive" content the cryptic link