|String or numeric expression that returns a key value that should be used to lookup a member of the dimension level.|
[Issue].[Issue].GetMemberByKey([Issue].CurrentHierarchyMember.get('Parent issue key')) returns a parent issue member for a sub-task issue.
If this function is used in a tuple code, sequence of members is very important - getmemberbykey function that references to a dimension should be before the dimension itself.
There is a similar function GetLinkedMember.