Graham's work-around is the quickest.

Otherwise, set up your numbering attached to styles as described at How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly. Then use space-before formatting for your top-level heading.

Note, you can modify the styles so that except for the numbering (and this spacing) the formatting is the same. You can set the default for the top-level so that when you press Enter, it will automatically produce level 2.