Unity WebGL cannot load remote js source?

I want to call Google Sign in API on WebGL build but it never run into onload callback, because of that I cannot call any api of that lib. Does anyone know this issue?

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        <script src="https://apis.google.com/js/platform.js?onload=initGoogleApi" async defer></script>
     <script>
              var auth2;
              function initGoogleApi() {
              gapi.load('auth2', function() {
                 console.log("initGoogleApi");
                 auth2 = gapi.auth2.init({
                     client_id: '379271268386-.......
                      scope: 'profile email'
                 })
               });
         }
             function onSignIn(){
                   auth2.signIn(); // error
             }
 <script>

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