I have similar issues any help please?
Unfortunately there is not.
The only MDX you can write in Tableau is within Tableau's Calculated Members dialog box.
Working with cubes and Tableau has its challenges. Ideally, I try to create any calculated members I may need directly in the cube because if I create them in Tableau, they will be query-scoped and not cache-able on the server. But I do end up having to create some in Tableau when push comes to shove, so I'm glad the feature is there.
I am facing the same issue, were you able to find any workaround?