If it’s a text-only font, click the down arrow next to the Font field box. Scroll through the list of fonts and choose one of the Bullet fonts, such as Wingdings, Wingdings 2, Wingdings 3, or ...

In the Symbol dialog box, find a symbol that looks like a checkbox. There are a number of options to choose from, but here's a good choice: In the "Font" drop-down, choose "Wingdings 2" and then ...