As Andrey informed you in your other thread, Aspose.Words for Java will include the feature to render floating content upon rendering. This will be avaliable as soon as autoporting for Java is ready which should be released sometime around the beginning next month.
In the mean time you could try to put the textbox in the header and set "Different first page header" option for the section. This should position the content properly on the page and the textbox should only appear on the first page.