Firstly, please note the reference function is only available in the References tab on the desktop version of Microsoft Word, it is not available through Word Online. For this, and other reasons including a lack of updating of the reference function, the library now tends to point students towards reference management tools that you can use with Word. The two that the library provides video tutorials for are Mendeley and Zotero. There are several others and the library does not endorse one over the other or subscribe to any of these (most of them have free versions).
If you do find the Word referencing tool works best for you please see the MS Word Referencing Tool - Guide and other support material listed below for details on how to automatically format in‐text citations and generate a bibliography or reference list in your Word document.
Important: Many editions of Word do not include Harvard as a referencing style option, computer services have created an article on how to add Harvard to the list of style options, see here.