How to enter in latex

The Apple Color Emoji font that contains this particular character doesn't encode its glyphs as regular characters and can't be used. See Emoji Characters. There seem to be very few fonts which contain these glyphs (the Enclosed Alphanumeric Supplement in Unicode).

On my machine (a Mac), only one font contains them: the Hiragino fonts. Here's an example that works for me (compile with LuaLaTeX, not XeLaTeX, which didn't work for me.)

\documentclass{article}
\usepackage{fontspec}
\usepackage{xcolor}
\newfontfamily\Hiragino{Hiragino Sans}[Color=red]
\begin{document}

{\Huge\Hiragino }
\end{document}

This sign in LaTeX it can work also without XeLaTeX. It is an alternative.

\documentclass{scrartcl}
\usepackage{xcolor}
\usepackage{tikz}
\newcommand{\Boo}[1]{
\begin{tikzpicture}[#1]
\node [fill=magenta, draw=magenta, rounded corners,minimum height = 12pt, minimum width = 12pt, inner sep = 1] at (10pt,10pt) {\textcolor{white}{\textbf B}};
\end{tikzpicture}
}
\begin{document}
We have \Boo{} a new symbol.

\end{document}

Maybe too classic, but ...

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[utf8]{xcolor}
\usepackage{newunicodechar}
\newunicodechar{}%
{{\fboxsep1pt\colorbox{black}{\color{white}\sffamily\bfseries B}}}
\newunicodechar{}%
{{\fboxsep1pt\colorbox{black}{\color{white}\sffamily\bfseries A}}}


\begin{document}

lck and white

\end{document}