Unfortunately we don't yet have a direct API for doing this, but it's planned for inclusion shortly.
In the meantime, you can send a transaction with metadata using the following calls in sequence:
preparelockunspent → createrawtransaction → appendrawmetadata → signrawtransaction → sendrawtransaction
Each one uses the output of the previous call. Please note there's a problem in the current alpha 9 with sending transactions which spend the output from preparelockunspent, so you should wait for alpha 10 to do this.