That's what I want too and I'm afraid such a feature is not implemented yet. You can try to open an issue, find a different client or switch to Mastodon because I know some of its clients have the feature (though it works unreliably sometimes).
It seems like it will filter out (block) any post containing your keyword anywhere in the post title, body, or URL. Not sure about comments, but it's a solid start.
We don't have this in jerboa, as I don't want to add "custom" features that are only available to jerboa, and not to every app. So this would need to be done in the back end first.
But also just block those communities, and if US spam is spilling over everywhere (like it does on reddit), then report those posts.
So what you're saying is that you want to block posts that contain the words "Biden", "Trump in their title? Or do you want to block posts that contain these words in their text body as well?