StellarFund.js

// load user account
const userAccount = await blockchain.loadAccount(userPubKey)

// add a payment operation to the transaction
const transaction = new stellarSdk.TransactionBuilder(userAccount)
.addOperation(
  stellarSdk.Operation.payment({
    destination: escrowPubKey, // user sends to escrow account
    asset: stellarSdk.Asset.native(), // native is XLM
    amount: '5'  // user pays 5 XLM
  })
)
.addOperation(
  stellarSdk.Operation.setOptions({
    source: escrowPubKey,   
    signer: {
      ed25519PublicKey: serverKeyPair.publicKey(), // add server as a signer on escrow account
      weight: 1
    }
  })
)
.addOperation(
  stellarSdk.Operation.setOptions({
    source: escrowPubKey,   
    signer: {
      ed25519PublicKey: userPubKey, // add user as a signer on escrow account
      weight: 1
    }
  })
)
.addOperation(
  stellarSdk.Operation.setOptions({
    source: escrowPubKey,
    signer: {
      sha256Hash: sha256HashOfPreimage, // hash taken from other chain. user has preimage
      weight: 1
    },
    masterWeight: 0, // escrow cannot sign its own txs
    lowThreshold: 2, // and add signing thresholds (2 of 3 signatures required)
    medThreshold: 2,
    highThreshold: 2
  })
)
.build()

// sign transaction
transaction.sign(serverKeyPair)

// https://www.stellar.org/developers/js-stellar-sdk/reference/examples.html
const fundEnvelope = transaction.toEnvelope().toXDR('base64')