So here's the biggest tip I can give you if you're using Mastodon's web interface...
Someone sent you a link to a post on an external site and you really wanna fave and boost it but you don't know how?
You can copy and paste the URL the search box and it'll show it in your local UI!
Most important undocumented feature of Mastodon's web UI IMO. You're welcome.
@cwebber might want to clarify you're talking about a link to a post on an external mastodon/fedi site