SponsorshipTx.js

const { sponsorship } = require('@waves/waves-transactions')

const seed = 'example seed phrase'

const params = {
  assetId: '4uK8i4ThRGbehENwa6MxyLtxAjAo1Rj9fduborGExarC',
  minSponsoredAssetFee: 100
}

const signedSponsorshipTx = sponsorship(params, seed)
The code above will create (but not send to the blockchain) a transaction:
{
  "id": "A",
  "type": 14,
  "version": 1,
  "senderPublicKey": "3SU7zKraQF8tQAF8Ho75MSVCBfirgaQviFXnseEw4PYg",
  "minSponsoredAssetFee": 100,
  "assetId": "4uK8i4ThRGbehENwa6MxyLtxAjAo1Rj9fduborGExarC",
  "fee": 100000000,
  "timestamp": 1575034734209,
  "proofs": [
    "42vz3SxqxzSzNC7AdVY34fM7QvQLyJfYFv8EJmCgooAZ9Y69YDNDptMZcupYFdN7h3C1dz2z6keKT9znbVBrikyG"
  ]
}