In that case, if you run the macro from a document containing the required header, you could simply delete:
Code:
StrTxt = InputBox("Text to add to headers? e.g. ""March 1, 2018""")
If Trim(StrTxt) = "" Then Exit Sub
and change:
.Sections(1).Headers(wdHeaderFooterPrimary).Range. InsertAfter StrTxt
to:
Code:
With .Sections(1).Headers(wdHeaderFooterPrimary).Range
.FormattedText = wdDocSrc.Sections(1).Headers(wdHeaderFooterPrimary).Range.FormattedText
.Characters.Last.Delete
End With