HTTP is for the generated output of building a repo, which has an index yaml. The repo you’re looking at says that they publish that at https://prometheus-community.github.io/helm-charts , so that’s what you should put in.
That worked, at least provisionally - now when I go to Apps & Marketplace all I see is "Loading…"and the whole screen is greyed out - it might have severely broken something behind the scenes, the entire “Apps & Marketplace” is completely unusable now.