ItsBarbara was telling us:
ItsBarbara nous racontait que :

It is much easier to have a userform (or an InputBox if the text to enter is
short and simple) by means of which the user can type the desired text.
Then, the code inserts the text in the header without opening it.

Here is a simple example with an InputBox (You will need a Bookmark named
"Test" in the main header of section #1.)

'_______________________________________
Dim strText As String

strText = InputBox("Type your text:", "Header Data")

'Will equal "" when user hits Cancel
If strText = "" Then Exit Sub

ActiveDocument.Sections(1).Headers(wdHeaderFooterPrimary) _
   .Range.Bookmarks("Test").Range.Text = strText
'_______________________________________