You need to configure the UserRegions role. Which Multidimensional Expressions (MDX) function should you use?

ANCESTOR() navigates upward in a hierarchy to retrieve a member at a specified level relative to a given member; it performs hierarchy traversal, not user identity lookup.

USERNAME() returns the authenticated user's identity in the form DOMAIN\username at query time. In a role's allowed member set expression, comparing USERNAME() against a security mapping table or a dimension attribute lets SSAS dynamically restrict which members each user can see, which is exactly what a UserRegions role requires.

FIRSTSIBLING() returns the first member among the siblings of a specified member; it is a positional navigation function with no relationship to user identity.

LEAD() returns the member that is a specified number of positions after a given member along a level; it is used for time-series navigation, not security filtering.

COUSIN() returns the member at the same relative position under a different parent as a specified child member; it is a structural navigation function unrelated to user identity.