You can test the user response to the MsgBox as follows:

Dim vAns As Variant

vAns = MsgBox("Do you want to add it to the list?", vbYesNoCancel)

Select Case vAns
Case Is = vbYes
'add it to the list...
Case Is = vbNo
'do something as if it was added to the list...
Case is = vbCancel
'do something else...
End Select

HTH
Garry