Function Kvartal(cel)
    Kvartal = Format(cel, "K" & "q" & " - " & "yyyy")

End Function

Sub Sample()
Dim currentSheet As Worksheet
Dim lRow As Long

Set currentSheet = ThisWorkbook.Sheets("Admin")

With currentSheet
    lRow = .Range("A" & .Rows.Count).End(xlUp).Row

    .Range("J2:J" & lRow).Formula = Kvartal
End With
End Sub