Classpaths in projects?

[NullPlane]

(WINDOWS, jdt.ls 1.9.0)
Need some help with projects.
I set up jdtls in my editor ecode and it works like a charm until I try to use a library from my university.
It came in a jar. When I open a single file it says non-project file - only syntax errors highlighted but when
I try to use my university's lib and open the whole folder as a project, it won't resolve anything outta there. I tried
adding a custom folder to %CLASSPATH% and putting it in there but no help.

I am forced to use ant so no maven etc.

Can I put it into a special folder with some name or anything? In the basedir I currently put it to .\res\dep but
obviously that is not found. It shall only be unpacked into lib before compiling but I'd love to do projects without
jdtls not finding my resources and marking them in red as errors as I'd need that to quickly find syntax problems.

How would I accomplish that? Is there some workaround or init value for such cases to give a folder with some jars?