unable to link the steam page on itch project page,
The format specified there is https://steamcommunity.com/sharedfiles/filedetails/?id=xxxxxx
But we want to link our store page http://store.steampowered.com/app/506090/ 
AFAIK there we do not have anything with the format steamcommunity.com/sharedfiled/filedetails

Am I missing something?

Sorry about the confusion, the example URL was from when Greenlight was a thing. I updated it to reflect a proper Steam page URL. Also, if you provide a Steam link on the edit page it will now show up under Links on the more information panel on your game's page.