Here are two different ways
The first adds to 1,2,3 the second only to selected slides
Sub Textboxnew()
Dim myTextBox As Shape
Dim mySld As Long
On Error Resume Next
'add to slide 1,2,3 change as required
For mySld = 1 To 3
With ActivePresentation.Slides(mySld)
Set myTextBox = .Shapes.AddTextbox _
(Orientation:=msoTextOrientationHorizontal, Left:=450, Top:=20, _
Width:=100, Height:=100)
myTextBox.TextFrame.TextRange.Text = "NEW"
myTextBox.TextFrame.TextRange.Font.Color.RGB = vbRed
myTextBox.TextFrame.TextRange.Font.Size = 20
End With
Next mySld
End Sub
Sub Textboxnew_Sel()
'adds only to selected slides
Dim myTextBox As Shape
Dim osld As Slide
Dim osldRng As SlideRange
On Error Resume Next
Set osldRng = ActiveWindow.Selection.SlideRange
For Each osld In osldRng
Set myTextBox = osld.Shapes.AddTextbox _
(Orientation:=msoTextOrientationHorizontal, Left:=450, Top:=20, _
Width:=100, Height:=100)
myTextBox.TextFrame.TextRange.Text = "NEW"
myTextBox.TextFrame.TextRange.Font.Color.RGB = vbRed
myTextBox.TextFrame.TextRange.Font.Size = 20
Next osld
End Sub