Change default check box style and formatting in Writer [closed]

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Unicode has corresponding characters - whether you can make use of them depends on what fonts you have installed.

There is U+25A1 WHITE SQUARE □ (and u+25a2 with rounded corners ▢ - U+2B1C WHITE LARGE SQUARE is rather new, so probably not included in your font ⬜), so probably a little too small for your usecase, and not meant for that anyway, more suited is: U+274F LOWER RIGHT DROP-SHADOWED WHITE SQUARE ❏ or U+2751 LOWER RIGHT SHADOWED WHITE SQUARE ❑ - and the favorite: U+2610 BALLOT BOX ☐

You can add autocorrect statement for those for example replace "[]" by the box character, and when you type <space>[]<space> it will replace it by the square character. Or an autoText, that you can then insert using bb<F3> for example (bb for "ballot box" - but you're free to use whatever shortcut you like)