If you're the mod of the community, you have to click on that post, then click the three dot button, in there you will find option for pinning. As for scheduling one, lemmy UI doesn't have this tool available yet and you either have to build one yourself, or contact @Naomikho@monyet.cc so she can use her bot to schedule one for you.

Edit: here's an example

That "📍Community" is the pin

Edit2: as for unpinning, i think it's because it's an admin(me) that pin it, only admin can unpin it(maaaaaybe). I already unpin the one in movies.