Can't find the latex code for the ⍎ (down tack jot) symbol

\documentclass{article}
\usepackage{stackengine,scalerel}
\newcommand\dtjot{\ThisStyle{\ensurestackMath{\stackengine{\dimexpr.6\LMpt+.2pt\relax}%
  {\SavedStyle\perp}{\SavedStyle\kern-.2pt\kern.08pt\circ}{O}{c}{F}{F}{L}}}
}
\begin{document}
$\dtjot\quad\scriptstyle\dtjot\quad\scriptscriptstyle\dtjot$
\end{document}

If you want the circle smaller,

\documentclass{article}
\usepackage{stackengine,scalerel,graphicx}
\newcommand\dtjot{\ThisStyle{\ensurestackMath{\stackengine{\dimexpr1\LMpt+.2pt\relax}%
  {\SavedStyle\perp}{\scalebox{.7}{$\SavedStyle\kern-.18pt\kern.08pt\circ$}}%
  {O}{c}{F}{F}{L}}}
}
\begin{document}
$\dtjot\quad\scriptstyle\dtjot\quad\scriptscriptstyle\dtjot$
\end{document}

You can superimpose \perp and \circ.

\documentclass{article}
\usepackage{wasysym}

\makeatletter
\DeclareRobustCommand{\APLdtj}{%
  \begingroup\m@th
  \vphantom{$\perp$}%
  \ooalign{\hfil\raisebox{0.2ex}{$\circ$}\hfil\cr$\perp$\cr}%
  \endgroup
}
\makeatother

\begin{document}

\APLbox \quad \APLdtj \quad \APLcomment

\end{document}

As you just used the Unicode symbol ⍎, you’re aware of it. You can use that in LaTeX, too. Load the fontspec package, compile in LuaLaTeX or XeLaTeX, and you can paste it into your source code. ^^^^234E also works, as does \char{"234e}, or you can define a \newcommand.

If your main font doesn’t contain that glyph, you can define the command in text mode like so:

\newfontfamily\symbolfont{DejaVu Sans}[Scale=MatchLowercase]
\newcommand\dtjot{{\symbolfont ⍎}}

In math mode, you would load unicode-math and use the range= option of \setmathfont if the main math font you want to use does not cover it. You could also put the text-mode command inside a \mbox.