Connecting to my smart contract on polygon testnet (Mumbai)

I created a smart contract (lets call it MyContract) and deployed it on Mumbai testnet. I used this tutorial to connect to MyContract. this is my code:

import maticjs from '@maticnetwork/maticjs';
const { POSClient,use } = maticjs;
import maticjsWeb3 from '@maticnetwork/maticjs-web3';
const { Web3ClientPlugin } = maticjsWeb3;
import HDWalletProvider from "@truffle/hdwallet-provider";
import * as dotenv from 'dotenv'
dotenv.config()

// install web3 plugin
use(Web3ClientPlugin);

const posClient = new POSClient();

let fromAddress = "<my private address (also the miner of the contract)>";

await posClient.init({
    network: 'testnet',
    version: 'mumbai',
    parent: {
      provider: new HDWalletProvider(process.env.MNEMONIC, process.env.MUMBAI_RPC_URL)
    },
    child: {
      provider: new HDWalletProvider(process.env.MNEMONIC, process.env.MUMBAI_RPC_URL),
      defaultConfig: {
        from : fromAddress
      }
    }
});

const erc20ChildToken = posClient.erc20("<MyContract address>");

const balance = await erc20ChildToken.getBalance(fromAddress);
console.log('balance', balance/10**18);


const result = await erc20ChildToken.stake(10);
console.log('result ', result)

as you can see in the last 2 rows I have a call erc20ChildToken.stake by which I intended to call the stake function of the contract, but I get this error:

TypeError: erc20ChildToken.stake is not a function

How can I access "special" functions that are not part of the standard ERC20 interface?