The deposit method can be used to deposit required amount from root token to child token.

const erc20RootToken = posClient.erc20(<root token address>, true);

//deposit 100 to user address
const result = await erc20Token.deposit(100, <user address>);

const txHash = await result.getTransactionHash();

const txReceipt = await result.getReceipt();

It might take some time to see the deposited amount on polygon chain. You can use isDeposited method for checking status.