This is not really a good subject line for you. Your subject line should be descriptive enough to indicate what you are looking for.
It should be
For i As Integer = 1 To 10
cboCategoryName.Items.Add(String.Format("{0} {1} {2}, frmSuppliers.txtFName.Text, frmSuppliers.txtMName.Text, frmSuppliers.txtLName.Text)
Next
Item collection takes 1 argument. If you are going to add a sentence to its item collection, you need to concat.