This will work in header-includes in your document's metadata, but not in header-includes in the metadata section of a defaults file file. The reason is that, as documented, values passed to --metadata are parsed as plain text, while metadata in a markdown file is parsed as markdown.

You could try 

variables:
  header-includes:
    - \usepackage{tikzsymbols}

Variable text is passed through unchanged (so no escaping).

> On Jul 3, 2023, at 1:46 AM, Oliver <news-WPTjrydoUPgeaOpM6FAJmQkbCANdLtlA@public.gmane.org> wrote:
> 
> Hi there
> 
> In a markdown document I can add for the LaTeX writer the following in the YAML metadata blocK:
> 
> ---
> header-includes:
>   - \usepackage{tikzsymbols}
> ...
> Then I can use in the document things like \Cooley[-3][cyan] and more. :-)
> 
> However I was not able to add this \usepackage declaration to a defaults file. I tried in my defaults file myarticle.yaml the following:
> 
> from: markdown+raw_attribute
> metadata:
>   documentclass: scrartcl
>   header-includes:
>     - \usepackage{tikzsymbols}
> but then I get the Latex error:
> 
> ! LaTeX Error: Missing \begin{document}.
> [...]
> l.46 \textbackslash
> Somehow the \usepackage{tikzsymbols} command is now „escaped“ so that the backslash get printed out... How can I avoid this and give the \usepackage declaration in the defaults file?
> 
> Thanks for any pointers!
> 
> Oliver
> 
> 
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