If the changing text is just the page number, then all you need to do is add a page field to the header along with your static text. Otherwise, you could use something like:
Code:
Sub Demo()
Application.ScreenUpdating = False
Dim p As Long, Shp As Shape, sWdth As Single
With ActiveDocument
With .PageSetup
sWdth = .PageWidth - .LeftMargin - .RightMargin - .Gutter
End With
Do While p < .ComputeStatistics(wdStatisticPages)
p = p + 1
Set Shp = .Shapes.AddTextbox(msoTextOrientationHorizontal, 0, 0, sWdth, 24, .GoTo(What:=wdGoToPage, Name:=p))
With Shp
.LockAnchor = True
.Line.Visible = False
.RelativeHorizontalPosition = wdRelativeHorizontalPositionMargin
.Left = wdShapeCenter
.RelativeVerticalPosition = wdRelativeVerticalPositionMargin
.Top = -18
With .TextFrame.TextRange.ParagraphFormat
.SpaceBefore = 0
.SpaceAfter = 0
.Borders(wdBorderBottom).LineStyle = wdLineStyleSingle
End With
End With
Loop
End With
Application.ScreenUpdating = True
End Sub