Two ways I can think of. One is to have a named range in the workbook
somewhere and set the RowSource to the name.

The other is to put your initialising code in the Workbook open event. (In
"This Workbook" in the VBE drop down the Left dropdown list  and select
"workbook" then the right drop down list and select open). This code will
fill the combobox  every time the workbook is opened.

Private Sub Workbook_Open()
With Worksheets("Sheet1").ComboBox1.
Value = "cc"
AddItem "A"
AddItem "B"
AddItem "C"
end with
End Sub