How to pass MUN value via URL to Report using Powercube datasource

[bstrahan]

I have a report Studio report based off a time partitioned PowerCube... 

I need to filter the report so it displays one specific category/member from a level in the cube that has 100,000 categories/members.

I'm using a MUN with a parameter in the Query item expression in order to filter the report to one category, chosen at runtime.. Query Item expression is like -

    [MyCube].[H1].[L1].[Name]->?category?

I have a tree prompt that allows me to navigate the hierarchy, select the category I want, and run the report. It works great, and the report runs lightening fast...

HOWEVER, I need to launch this report from another site using a URL.
I cannot get the syntax for my 'p_category' URL parameter to work.

So - my question is: How does one correct specify/format a MUN parameter (like my ?category? parameter) for passing via URL? 
Or is there another way to do what I want to do without sacrificing speed?

Bob

PS I have attempted to filter the report differently, using filter expressions in the query, or the 'filter()' function in the category query item, thereby allowing me to use a simple 'text value' parameter which I do know how to pass in the URL... these methods work  but are *very* slow..
With a filter, the reports takes 3 minutes.. With the MUN parameter (as above) it runs almost instantaneously.. a huge difference.