I think you have to enter the community in your url bar like this:
your.instance/c/fte@sopuli.xyz
which I think in your case would be:

discuss.tchncs.de/c/fte@sopuli.xyz

Try that out and get back to me. I am also brand new to the Fediverse, kind of feels like the blind leading the blind. But I’m sure we will figure it out.

edit: just realized what you’re asking. I know how to make a community link that can be accessed from any instance, but not a post link. I want to link the FAQ’s from my instance so that anyone can read it, but I’m still not sure how.