You should be able to use `-M header-includes='\usepackage{mypackage}'`
or put whatever LaTeX you want in a file and include it with `-H file.tex`
but then you can't have any header-includes in your file metadata.

In my latex template I have something like this near the top:

````
$for(use-packages)$
if(use-packages.name)$
\usepackage[$for(use-packages.opts)$$use-packages.opts$$sep$,$endfor$]{$
use-packages.name$}
$else$
\usepackage{$use-packages$}
$endif$
$endfor$
````

(I wrote this from memory so some details may be wrong.)

Now I can do this in my metadata:

````
---
use-packages:
  - somepackage
  - name: otherpackage
    opts: optname
  - name: thirdpackage
    opts:
      - firstoption
      - secondoption
---
````

and on the command line I can do:

````
-M use-packages=thispackage -M use-packages=thatpackage
````

Also if you are converting from some format other than Pandoc's Markdown
you can use the `--metadata-file=metadata.yaml` option and use

````````
header-includes:
  - |
    ```{=latex}
    \usepackage{somepackage}
    ```
````````

or custom template stuff and whatever metadata you need in that file.

Den fre 17 maj 2019 19:29ivo welch <ivowel-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> skrev:

>
> [Apologies.  I read the docs twice, but I could not find it.  This must be
> obvious.]
>
> I want to include in latex documents a '\usepackage{abc}`, so that my
> latex runs (or pdf runs) include it (following kpsewhich, of course).  I do
> not want pandoc to include the file itself and I do not want to set it in
> the yaml header.
>
> How?
>
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