Try the next way, please:
Dim list1 As Range
Application.EnableEvents = False
Dim validationFormula As String
Set list1 = Sheet3.Range("A1:A100")
validationFormula = Join(Application.Transpose(Application.index(list1, 0, 1)), ",")
With Sheet1.Range("C4:C2000").Validation
.Delete
.Add Type:=xlValidateList, Operator:=xlBetween, Formula1:=validationFormula
.IgnoreBlank = False
.InCellDropdown = True
End With
Application.EnableEvents = True
End Sub
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