How to pass date selections into custom column?

[Anmolgan]

am using below dax to pass a date selection from query 1 date table into the dax custom column, but it seems like the selection is not working, how can I manage to do this:

Column1 = var reference_date= SELECTEDVALUE(Query1[FullDateAlternateKey])

return DATE(YEAR(reference_date),MONTH(reference_date),1)