From the command line, open your wallet and type the following:
transfer 2 poloniex_address amount_sending payment_id
It should look like something like this:
transfer 2 44AFFq5kSiGBoZ4NMDwYtN18obc8AemS33DBLWs3H7otXft3XjrpDtQGv7SqSsaBYBb98uNbr2VBBEt7f2wfn3RVGQBEP3A 0.50 4a6ed4a061d23c660693a60844680454465f6afe615df1d6bf9f074877887e10
The above example sends 0.50 to Monero's donation address with a payment id of: 4a6ed4a061d23c660693a60844680454465f6afe615df1d6bf9f074877887e10