Compound signature from user and sponsor

signature.js

// Fetch the sponsor signature.
const sponsorSignature = await getSponosrSignature(
    '0x'+originAddress.toString('hex'),
    txBody
)
    
// Compose a combined signature of user + sponsor.
const sig = Buffer.concat([
    originSignature,
    Buffer.from(sponsorSignature, 'hex')
])

// Mount on the combined signature.
tx.signature = sig

// Convert the tx to raw format.
const rawTx = '0x' + tx.encode().toString('hex')

// Submit the raw transaction by hand to the test-net.
const url = 'https://sync-testnet.vechain.org/transactions'
fetch(url, {
    method: 'POST',
    headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json'
    },
    body: JSON.stringify({'raw': rawTx})
}).then(response => {
    response.text().then(r => {console.log(r)})
}).catch(err => {
    console.log('err', err)
})