Signing a Stellar transaction is identical to how we sign an Ethereum or Solana transaction with the exception that we need to pass in the Stellar specific RPC URL, chain ID, and method.
Before you submit a Stellar transaction, ensure that the Stellar account is funded and created on-chain. Failing to do so will result in an address not found error.
In the example we are going to update:
  • chainID to stellar:pubnet
  • rpcUrl to http://horizon.stellar.org
  • method to stellar_sendTransaction
  • params to a base64 encoded serialized Stellar transaction
Here is a short snippet of how to get the params using the stellar SDK. Note that in this example we assume the destination address is already created. For a complete example checkout this example repo: 

const axios = require('axios');
const StellarSdk = require('stellar-sdk');

const rpcUrl = 'https://horizon-testnet.stellar.org';

const server = new StellarSdk.Horizon.Server(rpcUrl);

const destinationAddress = "RECIPIENT_ADDRESS"

await server.loadAccount(destinationAddress);

const sourceAccount = await server.loadAccount("YOUR_STELLAR_ADDRESS");
const transaction = new StellarSdk.TransactionBuilder(sourceAccount, {
  fee: StellarSdk.BASE_FEE,
  networkPassphrase: StellarSdk.Networks.TESTNET, // Defaulting to testnet
})
  .addOperation(
    StellarSdk.Operation.payment({
      destination: destinationAddress, // example destination address
      asset: StellarSdk.Asset.native(), // Send XLM
      amount: '1', // amount to send
    }),
  )
  .setTimeout(30)
  .build();

// Serialize the transaction to XDR
const serializedTransaction = transaction.toXDR()

// // Convert the serialized Buffer to a Base64 string
const base64Transaction = serializedTransaction.toString('base64');

// // Sign the transaction
const signResponse = await axios.post(
  `${PORTAL_MPC_CLIENT_URL}/v1/sign`,
  {
    share: shares.ED25519.share,
    method: 'stellar_sendTransaction',
    rpcUrl,
    params: base64Transaction,
    chainId: 'stellar:testnet',
  },
  {
    headers: { Authorization: `Bearer ${clientApiKey}` },
  },
);
Below is an example curl command for signing a Stellar message: 
curl -X POST 'https://mpc-client.portalhq.io/v1/sign' \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer <clientApiKey>' \
  -d '{
    "share": "ED25519.share",
    "method": "stellar_sendTransaction",
    "params": "BASE64_ENCODED_TRANSACTION",
    "rpcUrl": "https://horizon-testnet.stellar.org",
    "chainId": "stellar:testnet"
  }'

​
Supported Methods and ChainIds

Methods:
  • stellar_sendTransaction
ChainIds:
  • stellar:pubnet
  • stellar:testnet