签名代码：

function addPreZero(num){
    var t = (num+'').length,
        s = '';
    for(var i=0; i<16-t; i++){
        s += '0';
    }
    return s+num;
}

function usdtSign(privateKey, utxo, feeValue, usdtValue, fromAddress, toAddress) {
    var txb = new bitcoin.TransactionBuilder();
    var set = bitcoin.ECPair.fromWIF(privateKey);
    const fundValue = 546;
    var usdtAmount = parseInt(usdtValue*1e8).toString(16);
    var totalUnspent = 0;
    for(var i = 0; i < utxo.length; i++){
        totalUnspent = totalUnspent + utxo[i].value;
    }
    const changeValue = totalUnspent - fundValue - (feeValue*1e8);
    if (totalUnspent < feeValue + fundValue) {
        console.log("Total less than fee");
        return constant.LessValue;
    }
    for(var i = 0; i< utxo.length; i++){
        txb.addInput(utxo[i].tx_hash_big_endian, utxo[i].tx_output_n, 0xfffffffe);
    }
    const usdtInfo = [
        "6f6d6e69",
        "0000",
        "00000000001f",
        addPreZero(usdtAmount)
    ].join('');
    const data = Buffer.from(usdtInfo, "hex");
    const omniOutput = bitcoin.script.compile([
        bitcoin.opcodes.OP_RETURN,
        data
    ]);
    txb.addOutput(toAddress, fundValue);
    txb.addOutput(omniOutput, 0);
    txb.addOutput(fromAddress, changeValue);
    for(var i = 0;i < utxo.length; i++){
        txb.sign(0, set);
    }
    return txb.buildIncomplete().toHex();
};

上面代码中UXTO的获取方式和比特币的一样。

案列代码：

var utxo ={
    "unspent_outputs":[
        {
            "tx_hash":"1bf1e457ac7572518cde36945e94728659dfae7fb2229411c1e13c085054c506",
            "tx_hash_big_endian":"06c55450083ce1c1119422b27faedf598672945e9436de8c517275ac57e4f11b",
            "tx_index":399167492,
            "tx_output_n": 2,
            "script":"76a91479b275dd5f136c241f3c28549b4c338177d5cb2188ac",
            "value": 1363462,
            "value_hex": "14ce06",
            "confirmations":0
        }
    ]
}

var privateKey = "私钥";
var usdtValue = 2;
var feeValue = 0.0002;
var fromAddress = "1C6UYrmbtvdi8dHZNnZD3YoVwit2mccSgw";
var toAddress = "1DefiYRCAD4wVS7rXwFkqhEn6R88EkSUnh";
var sign = usdtSign(privateKey, utxo.unspent_outputs, feeValue, usdtValue, fromAddress, toAddress);
console.log(sign);

签名成功后调用Omni区块链浏览器接口https://api.omniexplorer.info/v1/transaction/pushtx将其发到区块链网络即可