From your own wallet — OKX, Trust, MetaMask, Coinbase. No account, no SIGDA server. Open this page inside your wallet's browser, or use a desktop extension. Your message is recorded on Robinhood Chain as a readable event — it shows on Blockscout forever, and the chain proves you sent it.
send(to, body), a Message(from, to, body) event. Pass the deployed contract address as ?contract=0x… — there is no baked-in default since the address is assigned at deploy time (see contracts/script/DeployMessages.s.sol). Sending costs a few cents of Robinhood Chain ETH for gas.