Help with making a drop down list in many cells that allows me to select multiple options

I'm a translator trying to track and classify my work project. I want to create a column in which each cell has a list of subject categories (Legal, Business, Education etc) and I want it to be possible to be able to pick multiple subject categories. There are a number of "how to" guides online that give code but I have tried entering it where they tell me to and it's not having the desired effect. I have made the drop down list in one cell already. I just need to apply it to all cells in the same column (to infinity as I will keep adding jobs over the course of my career) and I need it to let me select multiple subject categories (I want to class a supplier contract under both business and legal for example.

I tried using this code ,changing only $C$2 to the box in my spreadsheet already containing the list ($K$2) This didn't work and I'm not sure why (although I have no coding background - I'm just trying to teach myself the basics: Thanks!

Private Sub Worksheet_Change(ByVal Target As Range)

'Code by Sumit Bansal from www.trumpexcel.com
' To Select Multiple Items from a Drop Down List in Excel

Dim Oldvalue As String
Dim Newvalue As String

On Error GoTo Exitsub
If Target.Address = "$C$2" Then
    If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
    GoTo Exitsub
    Else: If Target.Value = "" Then GoTo Exitsub Else
        Application.EnableEvents = False
        Newvalue = Target.Value
        Application.Undo
        Oldvalue = Target.Value
        If Oldvalue = "" Then
            Target.Value = Newvalue
        Else
            Target.Value = Oldvalue & ", " & Newvalue
        End If
    End If
End If
Application.EnableEvents = True

Exitsub:
Application.EnableEvents = True

End Sub