Hi,
try this code. I tested it on your string and got your desired result:
Private Sub Form1_Load(sender As Object, e As EventArgs)
Textbox1.Text = "Kumar, Ravi (Customer - Back office) - C088888"
Dim res As String() = Textbox1.Text.Split("-"C)
If res.Length >= 2 Then
Dim number As String = res(res.Length - 1).Replace(" ", "")
MessageBox.Show(number)
End If
End Sub