It’s under Functions: Lookup & Reference menu in Set Column Values dialogExamples:

idx(B==100) // Returns indices of values in B that equal 100

idx(B>=20 && B<=50) // Returns indices of values in B are between 20 and 50

idx(left(A,5)$ == "Chris") // Returns indices of values in A where first 5 letters are "Chris"

idx(diff(B)>1) // Returns indices in B where the difference between one value and the next value is greater than 1