Add Supervisor

\documentclass{report}

\usepackage{etoolbox}
\makeatletter
\newcommand{\supervisor}[1]{\gdef\@supervisor{#1}}
\patchcmd{\maketitle}% <cmd>
  {\@author}% <search>
  {\@author % <replace>
   \ifcsname @supervisor\endcsname \\ \@supervisor \fi}
  {}{}% <success><failure>

\renewcommand{\maketitle}{
\begin{titlepage}
\center
    {\Huge \@title} \\
    \@author \\
    \@supervisor \\
    \vfill
    \@date
\end{titlepage}
}
\makeatother

\author{Kajsa Eriksson}
\title{My Title}
\date{\today}
\supervisor{My supervisor}

\begin{document}

\maketitle

\end{document}