SSRS : How to give parameter values in URL Http requests?
Done.
Defining a parameter 'param2' whose values are integers and captions are strings, the point is the MDX query:
select {[Measures].[ID Distinct Count]} on COLUMNS,
  non empty filter({[DIM ANESTH].[ID].MEMBERS  },
  [DIM ANESTH].[ID].CURRENTMEMBER.MEMBERVALUE = @param2 ) on ROWS from BDD_PAT_ANESTH