there's no Current Context Site Collection url. But there is a Site url in the Workflow Context properties.
Maybe that is a good start and use a Replace action to replace a substring (site name) from the url.
Ignore the above.. I was in O365 land.
In the on premise workflow product, add a Build String action and then you can the Common property called Web Url. Then you can remove the Common property Site Name from that url using the fn-Replace inline function.
I hope that helps.
Hey Paul, did you find a way to get the site collection URL in the meantime? I have the same Problem. I think the Web URL gives back the site URL and not the site collection URL in which the workflow runs.
Best Regards, Thomas.