I want to fill it with the first row values of the active sheet . I came up
with this but isn't there a way to set the row source value?
Also, this is a listbox in a dialog that I want to load when the dialog
loads, is there a onload event or something that I can use to trigger this?

Thanks

Public Sub Load()

Dim Rng As Range
Const rw As Long = 1

With Sheets(ActiveSheet.Name)
If IsEmpty(ActiveSheet.Cells(rw, 1)) Then
Set Rng = .Cells(rw, 1)
Else
Set Rng = .Cells(rw, Columns.Count).End(xlToLeft)(1, 2)
End If
End With

Dim i As Integer
For i = 1 To Rng.Column - 1
ListBox_Describe.AddItem Cells(1, i)
Next i

End Sub