Also, if whoever is specifying this bullet is very picky, find out what font they want it to be pulled from. There are scads of hollow square or hollow very-nearly-square characters across multiple fonts that one could choose. If they have an existing document using that particular bullet, you can more easily determine precisely what character they want.
Otherwise, here are the steps to pick an hollow square as your bullet using the Wingdings font to choose the character from:
1. In your document, insert a bulleted list using ALT+H, U
2. The select bullet dialog comes up, hit D to activate Define bullet.
3. In the Define Bullet dialog, hit ALT+S for Symbol
4. In the Symbol Dialog, choose Wingdings from the dropdown for fonts, which should have focus upon entry to the dialog.
5. Hit ALT+C to get to the character number box, enter 111 in that box, which is the number for the hollow square character in Webdings.
6. Activate OK button out of Symbol dialog.
7. Activate OK button out of Define Bullet dialog.
8. Begin typing your first list item.
After having done this, the hollow square should appear first in the recently used bullets, which show up right after hitting ALT+H,U. You can select it directly from there after having defined it for the first time, or go through the whole process above (which is much faster than it seems once you've done it).
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore