The following MWE shows what appears to be a piece of paper folded. I want to indicate that the angle between the dashed lines is right angled by adding a simple square at M, such that the missing edges of the square are parallel to MD and MD2.
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[z={(0,1)},y={(1,0.6)},x={(3,-1)}]
\coordinate (A) at (0,2);
\coordinate (B) at (3,2);
\coordinate (C) at (3,0);
\coordinate (M) at (0.92,1.38);
\coordinate (D) at (1.85,2.77);
\draw[fill=white] (A)node{A}--(B)node{B}--(C)node{C}--cycle;
\path (M) ++(0,0,2.3) coordinate (D2);
\draw[fill=white,opacity=0.8](A)--(D2)--(C);
\draw[dashed](D2)node{D2}--(M)node{M}--(D)node{D};
\end{tikzpicture}
\end{document}