The issue here is not about user entering free value or the value not from the list.
The issue is if there is site A, B, C and user 1 should access only SIte A, user 2 should access only Site B, user 3 should access only Site C.
Now with my mandatory prompt not working for some reason I don't know why, User1 Can also select Site B and C and can see their details. User2 Can also select Site A and C and User3 Can also select Site A and B.
Row level secuirty in Universe is perfectly set right.