Run script in web whatsapp

I tried to run the next code with Gracemonkey:

var myWindow = null;
var setTiming = 5; //sec
var isMSGOUT = 0;
var msgs = [];

function us_openWAwindows() {
    myWindow = window.open("", "_blank", "toolbar=no,scrollbars=no,resizable=yes,top=500,left=500,width=400,height=400");
    myWindow.document.body.innerHTML = "<p>Plz wait, loading...</p>";
   
    //document.getElementById("startButton").disabled = true;
   
    var main = document.getElementById("main").getElementsByClassName("message-list");
    var list = main[0].getElementsByClassName("msg");
    var i;
    for (i = 0; i < list.length; i++) {
        list[i].innerHTML += '<button type="button" class="addButton" onclick="javascript:addNewMsg(this.parentElement)">Add</button>';
    }
   
    us_loadNewMsg();
}

function us_loadNewMsg() {
    if (msgs.length > 0)
    {
        myWindow.document.body.innerHTML = "<h3>" + msgs[0][0] + "</h3>";
       
        if ( msgs[0][2] == 1 )
        {
            myWindow.document.body.innerHTML += "<img id='showIMG' src='" + msgs[0][1].src + "' style='height:" + msgs[0][1].height +"'>";
        }
        else
        {
            myWindow.document.body.innerHTML += msgs[0][1];
        }
       
        msgs.shift();
        isMSGOUT=0;
    } else if (isMSGOUT == 0) { myWindow.document.body.innerHTML += "<br /><br />add new"; isMSGOUT=1;}
    us_timing();
}

function us_timing() {
    setTimeout(loadNewMsg, setTiming*1000);
}

function us_addNewMsg(thisMSG) {
    if ( thisMSG.getElementsByClassName("message-text").length > 0 )
    {
        msgs.push ([ thisMSG.getElementsByClassName("message-author")[0].getElementsByClassName("emojitext")[0].innerHTML,
                        thisMSG.getElementsByClassName("message-text")[0].getElementsByClassName("emojitext")[0].innerHTML,
                        0
                    ]);
    }
    else if ( thisMSG.getElementsByClassName("image-thumb").length > 0 )
    {
        msgs.push ([ thisMSG.getElementsByClassName("message-author")[0].getElementsByClassName("emojitext")[0].innerHTML,
                thisMSG.getElementsByClassName("image-thumb")[0].getElementsByClassName("image-thumb-body")[0],
                1
            ]);
    }
   
    thisMSG.getElementsByClassName("addButton")[0].disabled = true;
}

document.body.innerHTML += '<div style="position:absolute;bottom:20px;right:20px;z-index:80;border:1px black solid"><button type="button" id="startButton" onclick="javascript:us_openWAwindows()">Start</button></div>';

however, it's add the button, but cannot click on it...
I tried bypass it and run us_openWAwindows, but it won't work


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