In order to utilize the Elastos ETH Sidechain on mainnet of course you need to transfer ELA from the mainchain to the ETH sidechain.
There are a few ways to do this, but the most straightforward is using the
ela-cli tool bundled with the mainchain ELA full-node repository, this CLI tool has cross-chain functions
you can use to send ELA to and from the ETH sidechain.
./ela-cli wallet buildtx crosschain --from [your address] --saddress XVbCTM7vqM1qHKsABSFH4xKN1qbp7ijpWf --to [your ELAETHSC address] --amount 5 --fee 0.001
Where –saddress is the crosschain genesis address found by getting the gensis hash (block 0) and then running
/ela-cli wallet crosschainaddr [genesis hash]
Then sign and send this as usual.
./ela-cli wallet signtx --file to_be_signed.txn
./ela-cli wallet sendtx --file ready_to_send.txn