Special Character boxtime

⊠ is Unicode character U+22A0, and so we can input it in Mathematica by typing the following:

\:22A0

anywhere in a notebook.

This functionality is documented in tutorial/CharacterCodes.

As noted by Somos and in the comments to this answer, the display of these characters is font-dependent. The codepoint is valid, but if the font Mathematica is using doesn't have a glyph for the character, it will display as a small box with a question mark inside of it. You may need to play with your font settings to get the character to display correctly.

The code \:22A0 will produce the Unicode character for BOX TIMES but the font used for output must support the Unicode character and not all of them do, otherwise an empty "Box" (or something similar) will appear. For example, the font "Lucida Sans Unicode" displays the character correctly. In any case, this does not affect the internal string contents even if it is not displayed as you expected.

For another example, the code ToCharacterCode["a⊠b"] will return {97, 8864, 98} and the reverse code FromCharacterCode[{97, 8864, 98}] will return the original string.