burnproof.js

const matic_raw = require("./maticjs")
const config = require("./config.json")
const ext_mngr = require("./maticjs/lib/common/ExitManager")
// console.log(ext_mngr)
const root_chain = require("./maticjs/lib/root/RootChain.js")
// console.log(root_chain)
const { default: ExitManager } = require("./maticjs/lib/common/ExitManager")
const { default: RootChain } = require("./maticjs/lib/root/RootChain.js")
const params = {
    network: "testnet",
    version: "mumbai",
    maticProvider: config.MATIC_PROVIDER,
    parentProvider: config.PARENT_PROVIDER,
    posRootChainManager: config.ROOTCHAIN_MANAGER_PROXY,
    posERC721Predicate: config.ERC721_PREDICATE_PROXY,
    posERC1155Predicate: config.ERC1155_PREDICATE_PROXY,
}
async function exec() {
    const pos_client = new matic_raw.MaticPOSClient(params)
    // console.log(pos_client)
    const mgr = new ExitManager(RootChain, params, pos_client)
    const temp = mgr.web3Client.posRootChainManager.exitManager
    // console.log(temp)
    const payload = await temp.buildPayloadForExit(
        "0x4756b76a9611cffee3d2eb645819e988c34615621ea256f818ab788d81e1f838", // BURN TX: Change this to your hash
        "0x8c5261668696ce22758910d05bab8f186d6eb247ceac2af2e82c7dc17669b036") // MESSAGE_SENT_EVENT_SIG: Do not change this
    console.log(payload)
}
exec()

maybe more straightforward to just call posClient.exitUtil.buildPayloadForExit(txhash, sig)