This page is deliberately on a different origin from the queue.
That is the whole point of it: the interesting parts of the flow — whether
/compose accepts the post, and what happens to a session cookie that
lives somewhere else — only misbehave across origins, and never show up when the
form and the service share a host.
Submitting hands you to the queue, which previews what you sent and asks you to sign in before anything is recorded. Nothing is queued by pressing this button.
A 403 from /compose means this site's origin is not in the queue's ALLOWED_ORIGINS. Add it there and redeploy the queue — that check exists so an arbitrary site cannot stage submissions in someone's tab.