If the length of the string remains constant:
Dim WorkOrderNum = "WO123000"
Dim testStr As String = String.Format("{0}-{1}", WorkOrderNum.SubString(0, 2), WorkOrderNum.SubString(2, 6))
Console.WriteLine(testStr)
'outputs WO-123000
Although I'm sure there's a better way of doing this which should include error handling etc this was just a demo!