Substr(GetBookmarkableURL(),0, Index(GetBookmarkableURL(), GetPageName(),startIndex:,searchFromEnd:,ignoreCase:))
I'll add a vote. Hector's solution worked for me in the interim.
This Substr don't work if you are using Chrome (version: 64.0.3282.137) with Android (version: 8.0.0). But the same test works on Chrome with iOS.