function setCookie(name,value,days) {
    var expires = "";
    if (days) {
        var date = new Date();
        date.setTime(date.getTime() + (days*24*60*60*1000));
        expires = "; expires=" + date.toUTCString();
    }
    document.cookie = name + "=" + (value || "")  + expires + "; path=/";
}

var pageID = window.__page_id;
var desiredVariation = "A"; // Replace "A" with the variation you want to force load

setCookie('instapage-variant-' + pageID, desiredVariation, 1);