Please keep that 10 customers name in a separate sheet called Top Clients and mention the Top client names in Column-A.
In your entry sheet do right click on sheet tab and select view code and paste the below code.
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("A:A")) Is Nothing Then Exit Sub
Dim vFound As Variant
vFound = Application.Match(Target.Value, Sheets("Top Clients").Range("A:A"), 0)
If IsNumeric(vFound) Then MsgBox "This Client is in Top Clients List", vbQuestion, "Discount Reminder"
End Sub
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