uses
 ComObj;
procedure TForm1.Button1Click(Sender: TObject);
var
 WordApp: OLEvariant;
begin
 Screen.Cursor := crHourglass;
 try
  // Create Word Instance
  WordApp := CreateOleObject('Word.Application');
 except
  ShowMessage('Cannot start MS Word.');
  Screen.Cursor := crDefault;
  Exit;
 end;
 try
  // Open a Word Document
  WordApp.Documents.Add(Template := 'C:\TestDoc.doc');
  // Show Word
  WordApp.Visible := True;
  // Check if FormField exists and asign your text
  if WordApp.ActiveDocument.Bookmarks.Exists('YourFormFieldName') then
  WordApp.ActiveDocument.FormFields.Item('YourFormFieldName').Result := 'Your Text';
 finally
  WordApp := Unassigned;
  Screen.Cursor := crDefault;
 end;
end;