Not one message — a live window onto their box, showing whatever they send next. They need no account and nothing installed. You keep it open as long as you want it, and closing it is closing the page.
Sending one instead? Open the web app → (needs your Google sign-in)
Make up a signal, hand it over, and type the portal key they gave you beforehand. Why two?
Paste the signal you were given and the portal opens wherever you are — this device does not need to be the one that made it. You will need the portal key too.
The signal is minted here and travels to them; the portal key was agreed between you beforehand and is never sent. A portal needs both, so whoever catches the signal on its way has one half of two. Nothing flows back to them — a portal is a window, not a conversation.
Keep the signal and you can pick the portal up on any device — the two halves together are the portal, and neither this page nor the directory holds the key.