Mendeley Cite is a Microsoft Word plugin that allows you to cite from Mendeley in Word documents.

Install Mendeley Cite in Office 365 here: https://www.mendeley.com/reference-management/mendeley-cite

Note that Mendeley Cite only works on the desktop version of Office 365 -- and not in the web-based version.  You will see Mendeley Cite (once installed) under the References tab in your desktop version of Microsoft Word.