tx-frame.cs

// add a header
textControl1.Sections.GetItem().HeadersAndFooters.Add(HeaderFooterType.Header);

// retrieve the inserted header
HeaderFooter hed = textControl1.Sections.GetItem().HeadersAndFooters.GetItem(
    HeaderFooterType.Header);

// create a new TextFrame
TextFrame frame = new TextFrame(new Size(3000, 3000));

// add the TextFrame to the header
hed.TextFrames.Add(frame, 
    textControl1.InputPosition.Page, // fixed on page
    new Point(0, 2000), 
    TextFrameInsertionMode.BelowTheText | TextFrameInsertionMode.FixedOnPage);

// add text to TextFrame
frame.Selection.Text = "New TextFrame";