Custodial account transactions are unique as these users cannot sign transactions using their web3 wallets. To ensure security and align with the characteristics of web3 transactions, we have developed a special application called the Custodial Signer. This application securely handles the transaction signing process by communicating with a server specifically designed to manage custodial account transactions.
To use custodial accounts to complete transactions, requires users to implement a popup window and communicate with the custodial signer via post messages to obtain the signature. Then, the obtained signature is used to send the transaction to the blockchain.
Refer: getSignatureFromCustodialSigner to get signature on web and native(server) clients.
Refer: transactions for signEthTransaction, signRootTransaction and sendTransaction.