The trick is to use a frame instead of a textbox. Here is the simplest way to insert a frame in Word 2010/2013.
Alternatives include using a Table rather than a textbox.
http://www.extendoffice.com/documents/word/733-word-insert-frame.html#a2
(here is the official support suggestions:
https://support.clarivate.com/Endnote/s/article/EndNote-Citation-numbers-out-of-order?language=en_US