Start an async Make Transaction operation. With structured Destination (and optional EVM / chain-specific fields), creates, signs, and broadcasts the transaction when complete. With RawTransaction (and optional TxHash for Canton prepared submission), MPC signing only—the operation does not broadcast; the response includes SignedTransaction and the integrator must submit the signed bytes to the network. Requires Source as AddressSpec and CAIP19. Status can be queried with GET /operations/id/{operationID}/status.