Create OP_RETURN tx with Bitcoinjs-lib
Here:

var bitcoin = require('bitcoinjs-lib')

data = new Buffer("Melons.")

var tx = new bitcoin.TransactionBuilder()
tx.addInput("aa94ab02c182214f090e99a0d57021caffd0f195a81c24602b1028b130b63e31", 0)
tx.addOutput("some address", 15000)
ret = bitcoin.script.compile(
  [
    bitcoin.opcodes.OP_RETURN,
    data
  ])
tx.addOutput(ret, 0)
key = bitcoin.ECPair.fromWIF("L1uyy5qTuGrVXrmrsvHWHgVzW9kKdrp27wBC7Vs6nZDTF2BRUVwy")
tx.sign(0, key)
console.log(tx.build().toHex())


That should do it.

Also, if you have more than 40 bytes of data, your transaction will be non-standard.