part1.html

<center>
    <div class="fb-messenger-checkbox" origin="" page_id="PAGEID" messenger_app_id="APPID" user_ref="" prechecked="true" allow_login="true" size="large"></div >
</center> 

    <script> window.fbMessengerPlugins = window.fbMessengerPlugins || {
        init: function() {
            FB.init({
                appId: "APPID",
                xfbml: true,
                version: "v8.0"
            });
        },
        callable: []
    };
    window.fbMessengerPlugins.callable.push(function() {
        var ruuid, fbPluginElements = document.querySelectorAll(".fb-messenger-checkbox[page_id='PAGEID']");
        if (fbPluginElements) {
            for (i = 0; i < fbPluginElements.length; i++) {
                ruuid = 'cf_' + (new Array(16).join().replace(/(.|$)/g, function() {
                    return ((Math.random() * 36) | 0).toString(36)[Math.random() < .5 ? "toString" : "toUpperCase"]();
                }));
                fbPluginElements[i].setAttribute('user_ref', ruuid);
                fbPluginElements[i].setAttribute('origin', window.location.href);
                window.confirmOptIn = function() {
                    FB.AppEvents.logEvent('MessengerCheckboxUserConfirmation', null, {
                        app_id: "APPID",
                        page_id: "PAGEID",
                        ref: "checkbox",
                        user_ref: ruuid
                    });
                };
            }
        }
    });
    window.fbAsyncInit = window.fbAsyncInit || function() {
        window.fbMessengerPlugins.callable.forEach(function(item) {
            item();
        });
        window.fbMessengerPlugins.init();
    };
    setTimeout(function() {
        (function(d, s, id) {
            var js, fjs = d.getElementsByTagName(s)[0];
            if (d.getElementById(id)) {
                return;
            }
            js = d.createElement(s);
            js.id = id;
            js.src = "//connect.facebook.net/en_US/sdk.js";
            fjs.parentNode.insertBefore(js, fjs);
        }(document, 'script', 'facebook-jssdk'));
    }, 0); </script>
   

part2.html

<script>

Array.from(document.querySelectorAll("#register")).forEach(function(

  btn

) {

    btn.addEventListener("click", function() { window.confirmOptIn(); });

});

</script>