Bridging your custom ERC-20 token using the Standard Bridge

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
 
import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";
import { IOptimismMintableERC20 } from "https://github.com/ethereum-optimism/optimism/blob/v1.1.4/packages/contracts-bedrock/src/universal/IOptimismMintableERC20.sol";
 
contract MyCustomL2Token is IOptimismMintableERC20, ERC20 {
    /// @notice Address of the corresponding version of this token on the remote chain.
    address public immutable REMOTE_TOKEN;
 
    /// @notice Address of the StandardBridge on this network.
    address public immutable BRIDGE;
 
    /// @notice Emitted whenever tokens are minted for an account.
    /// @param account Address of the account tokens are being minted for.
    /// @param amount  Amount of tokens minted.
    event Mint(address indexed account, uint256 amount);
 
    /// @notice Emitted whenever tokens are burned from an account.
    /// @param account Address of the account tokens are being burned from.
    /// @param amount  Amount of tokens burned.
    event Burn(address indexed account, uint256 amount);
 
    /// @notice A modifier that only allows the bridge to call.
    modifier onlyBridge() {
        require(msg.sender == BRIDGE, "MyCustomL2Token: only bridge can mint and burn");
        _;
    }
 
    /// @param _bridge      Address of the L2 standard bridge.
    /// @param _remoteToken Address of the corresponding L1 token.
    /// @param _name        ERC20 name.
    /// @param _symbol      ERC20 symbol.
    constructor(
        address _bridge,
        address _remoteToken,
        string memory _name,
        string memory _symbol
    )
        ERC20(_name, _symbol)
    {
        REMOTE_TOKEN = _remoteToken;
        BRIDGE = _bridge;
    }
 
    /// @custom:legacy
    /// @notice Legacy getter for REMOTE_TOKEN.
    function remoteToken() public view returns (address) {
        return REMOTE_TOKEN;
    }
 
    /// @custom:legacy
    /// @notice Legacy getter for BRIDGE.
    function bridge() public view returns (address) {
        return BRIDGE;
    }
 
    /// @notice ERC165 interface check function.
    /// @param _interfaceId Interface ID to check.
    /// @return Whether or not the interface is supported by this contract.
    function supportsInterface(bytes4 _interfaceId) external pure virtual returns (bool) {
        bytes4 iface1 = type(IERC165).interfaceId;
        // Interface corresponding to the updated OptimismMintableERC20 (this contract).
        bytes4 iface2 = type(IOptimismMintableERC20).interfaceId;
        return _interfaceId == iface1 || _interfaceId == iface2;
    }
 
    /// @notice Allows the StandardBridge on this network to mint tokens.
    /// @param _to     Address to mint tokens to.
    /// @param _amount Amount of tokens to mint.
    function mint(
        address _to,
        uint256 _amount
    )
        external
        virtual
        override(IOptimismMintableERC20)
        onlyBridge
    {
        _mint(_to, _amount);
        emit Mint(_to, _amount);
    }
 
    /// @notice Prevents tokens from being withdrawn to L1.
    function burn(
        address,
        uint256
    )
        external
        virtual
        override(IOptimismMintableERC20)
        onlyBridge
    {
        revert("MyCustomL2Token cannot be withdrawn");
    }
}

    /// @notice Prevents tokens from being withdrawn to L1.
    function burn(
        address,
        uint256
    )
        external
        virtual
        override(IOptimismMintableERC20)
        onlyBridge
    {
        revert("MyCustomL2Token cannot be withdrawn");
    }

_BRIDGE:      "0x4200000000000000000000000000000000000010"
_REMOTETOKEN: "<L1 ERC-20 address>"
_NAME:        "My Custom L2 Token"
_SYMBOL:      "MCL2T"