You should anchor the shape (or text box) to the first page header, not to a paragraph in the main body of the document.
In the attached document, I have done the following: First I cut the shape from the main body. Then I double-clicked the header area. After it got activated, I selected the "Different first page" option on the ribbon. I then pasted clipboard contents, saved and closed the file.
For more on letterheads, see the article that Charles linked to.
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Stefan Blom
Microsoft Word MVP
Microsoft 365 apps for business
Windows 11 Professional
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