You can include LaTeX code with the LaTeX-Preambule: AMC-TXT general option.

I was playing around with lines like this

LaTeX-Preambule: pdfpages
$\includepdf[pages=-,landscape=true]{Rechenaufgaben.pdf}$

(well to be specific this one was for trying to include some full pdf pages to the document)

The header code is something like

\begin{center}
\vspace*{.1cm}
\hspace*{.8cm}
\begin{minipage}{0.2\linewidth}
\begin{center}
\includegraphics[scale=1.2]{media/buw.PNG}\\
\end{center}
\end{minipage}
\hspace*{.1cm}
\begin{minipage}{0.4\linewidth}
\begin{center}
\bf Prof. Dr.-Ing. asfsfag\\
\small\bf Klausur\\\large \glqq Easfsaf\\\small im Wintersemester 2018/2019\\
\end{center}
\end{minipage}
\hspace*{-.8cm}
\begin{minipage}{0.3\linewidth}
\begin{center}
\includegraphics[scale=0.3]{media/Logo_DE.png}\\
\end{center}
\end{minipage}\\
\makebox[\linewidth]{\rule{0.62\paperwidth}{0.4pt}}
\end{center}

Do you mind giving an example how the command should look like in AMC-TXT? I love the documentation till the point that it gives not many examples for the AMC-TXT Options like

LaTeX:

Set this option to 1 if you want to use LaTeX commands in your texts. This allows for example to insert mathematical formulas, like $\sqrt{a+b}$. If 0 (default), all your texts will be written unmodified.
LaTeX-Preambule:

Give commands you want to be added to the LaTeX preambule (for example \usepackage commands).
LaTeX-BeginDocument:

Give commands to be inserted at the beginning of the LaTeX document environment (for example macro definitions).

LaTeX-Preambule: includes some LaTeX code once in the document preamble, to define macros or set options, like:

LaTeX-Preambule: \fancyfoot[C]{ My footer }

LaTeX-Preambule: \AMCsetFoot{ My footer }

I tried the first one without success some days ago (no errors but also no effect)

But the second one works like a charm! thanks! Any quick tip to let it display pagenumbers?

So the part for the recent page number \thepage from https://project.auto-multiple-choice.net/boards/2/topics/8391 works. But not the total page count part.

\fancyheader sadly isn't working.. there is no \AMCsetHead - or is there?

there is no \AMCsetHead - or is there?

You're right: there is not.