The "enter the username @ domain you want to act from" feature doesn't seem to exist on Pleroma (or at least I can't find it anywhere), which is a bit annoying when you want to boost/favorite a post from there.
Though, grabbing a post's URL and pasting it on Mastodon's search box seems to work at least.
Welcome to your niu world ! We are a cute and loving international community Ｏ(≧▽≦)Ｏ !
We are a moderated instance, that aren't supporting harassment nor hateful speech. But we aren't a "safe" space, we won't prevent you to interact with instances that aren't respecting our rules.
"Be conservative in what you send and liberal in what you receive." - Netiquette
The main language used here is English, but for most of us this isn't our main language, so it's a great place to learn!