Mastodon servers running the experimental ("nightly") version of Mastodon may have quote-tooting now active.
If it is active, it should work by clicking a post's boost 馃攣 button, which offers the option to quote if the account has opted into quoting.
Don't worry if it's not yet active for you, this is just on experimental Mastodon as the feature is still being tested.
Quoting will be released in the next server software version (v4.5.0).
(Thanks @NatureMC for info! 馃檹 )