Can I use a URL parameter to open a thread at the first message?

No. Thread timelines in Plain always open scrolled to the latest message. You cannot change this with a URL parameter, a user setting or a workspace setting. There is no scroll=top, anchor or similar query parameter on thread URLs.

To read a thread from the beginning, scroll up after opening it. Plain loads earlier timeline entries as you scroll.

If you know the ID of a specific timeline entry, you can link straight to it with the ?entryId=<timelineEntryId> query parameter. This only works when you already have the entry ID, so it is not a general way to open a thread at the top.