ロジック

クリップボードの値を、新規のShapeに値貼り付け
作成したShapeから文字列を取得
作成したShapeを削除

Public Sub Test() As Slide
    Dim ActiveSlide As Powerpoint.Slide

    Dim sText As String 


    Set ActiveSlide =  ActivePresentation.Slides.FindBySlideID(ActivePresentation.Windows(1).Selection.SlideRange.SlideID)
    On Error GoTo err_
    
    With ActiveSlide.Shapes.PasteSpecial(ppPasteText)
        sText = .TextFrame.TextRange.Text
        .Visible = False
        Call .Delete
    End With

    On Error GoTo 0

    Set ActiveSlide = Nothing

    MsgBox sText 
err_:
End Function