If your linked margin brokerage account already has sufficient funds, there is no need to make additional transfers to separately fund futures trading.
The minimum margin requirement for futures positions held overnight will be automatically transferred to your E*TRADE futures account, including commission and fees, and any deficiency funds required to satisfy margin calls.
Conversely, any excess margin and available cash will be automatically transferred back to your margin brokerage account where SIPC protection is available.
Please note: The above applies only to linked margin brokerage accounts at E*TRADE. For unlinked E*TRADE accounts, there is no automatic transfer of minimum margin requirements or deficiency funding. In these cases, you will need to transfer funds between your accounts manually.