Function to specify the member of a calc member block

@member(@SUBSTRING({Period},0,2))

I would like to pass in a run time prompt that I can then use to derive the member to be used as the calc member block. So picture this:

RTP Name: {Period}

RTP Value: P01-FY24

Planning biz rule:

…
 @member(@SUBSTRING({Period},0,2))
  (
    IF(….)
      x = y;
    ENDIF
)

In theory the "@member(@SUBSTRING({Period},0,2)) " returns "P01" as a member to be used as the calc member block. The function works in a FIX just not when specifying a member for a calc member block. Is there an alternative way (other than groovy)? Thx