In order to participate in BAT ICO you should send your ETHers on following address with data.
The simplest way is sending transaction using MyEtherWallet.
When block number in Ethereum network will reach
3,798,640 you can send your transaction as follows.
Fill all fields as follows.
Amount to Send: amount in ethers you want to send to BAT project
200 000 (but bigger value is making participation more sure to happen)
Sign transaction & confirm it.
If you send your transaction before destination block nothing will happen and your Ethers will stay in your wallet.
All transactions send before block
3,798,640 should throw error (red message) and will not affect your ETH balance except small amount burned as GAS.