\documentclass[12pt,a4paper]{scrlttr2}

\newcommand{\dir}{~/projects/latex} % IMPORTANT: No trailing slashes!
\input{\dir/include.tex}
\load{letter}

% Subject
\setkomavar{subject}{Test}
\begin{document}\raggedright
\begin{letter}{Company \\ Name \\ Address \\ CH-Place}
    \opening{Intro}

    Test

    \closing{Kind regards}
\end{letter}
\end{document}
