The LaTeX command for boxplus (⊞) is \boxplus.
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Write \boxplus inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amssymb package, so add \usepackage{amssymb} to your preamble.
As a binary operator, \boxplus gets a medium space on each side; at the start of a formula LaTeX reads it as a sign and drops the space. With unicode-math under XeLaTeX or LuaLaTeX, typing ⊞ in a formula gives the same symbol.
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Binary operators in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \boxplus in math mode after adding \usepackage{amssymb} to your preamble. For example, $a \boxplus b$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.