
\documentclass{article}

\newcommand\thing[2][\DefaultOpt]{%
  \def\DefaultOpt{#2}%
  optional arg: #1,  mandatory arg: #2%
}

\begin{document}
\thispagestyle{empty}
\thing{manda}% #1=#2

\thing[opti]{manda}% #1="opti"
\end{document}
