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Signing a Solana transaction is identical to how we sign an Ethereum transaction with the exception that we need to pass in the Solana specific RPC URL, chain ID, and method. In the example we are going to update:
  • chainID to solana:4uhcVJyU9pJkvQyS88uRDiswHXSCkY3z
  • rpcUrl to https://api.testnet.solana.com
  • method to sol_signAndConfirmTransaction
  • params to a base64 encoded serialized Solana transaction
Here is a short snippet of how to get the params using @solana/web3.js, for a complete example checkout this example repo: 
  const {
    Transaction,
    SystemProgram,
    LAMPORTS_PER_SOL,
  } = require('@solana/web3.js');

  const tx = new Transaction({
    recentBlockhash: blockhash,
    feePayer: feePayerAddress,
  }).add(
    SystemProgram.transfer({
      fromPubkey: fromPublicKey,
      toPubkey: toPublicKey,
      lamports: LAMPORTS_PER_SOL * 0.01, // 0.01 SOL
    }),
  );

  // Serialize the transaction to a Buffer
  const serializedTransaction = tx.serialize({ requireAllSignatures: false });

  // 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: share,
      method: 'sol_signTransaction',
      params: base64Transaction,
      chainId: 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp',
    },
    {
      headers: { Authorization: `Bearer ${clientApiKey}` },
    },
  );
Below is an example curl command for signing a Solana 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": "sol_signTransaction",
    "params": "AQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAEDqF/lgjmUoeVVHb3ObBSh4S4W1vrG333tiaGvUzWHYDLkuw5hIt53j8IqHX8I+gj28DXUsAE3bgbROXDHpUdhhAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAlNc9WKVngB8ANy0SFkCbEM6i7nZKuqAgsFEvoZkO4/ABAgIAAQwCAAAAECcAAAAAAAA=",
    "chainId": "solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp"
  }'