Edit in JSFiddle

var json = {
  questions: [{
    type: "dropdown",
    renderAs: "select2tagbox",
    isRequired: true,
    choicesByUrl: {
      url: "https://restcountries.eu/rest/v1/all"
    },
    name: "countries",
    title: "Please select all countries you have been for the last 3 years."
  }],
  completedHtml: "<p><h4>Thank you for the sharing information wiht us!</h4></p><p>You've been in the following countries: <b>{countries}</b></p>"
}

Survey.defaultBootstrapCss.navigationButton = "btn btn-green";
Survey.Survey.cssType = "bootstrap";

var survey = new Survey.Model(json);

survey.onComplete.add(function(result) {
  document.querySelector('#result').innerHTML = "result: " + JSON.stringify(result.data);
});

$("#surveyElement").Survey({
  model: survey
});
<div id="surveyElement"></div>

<div id="result"></div>
.btn-green {
  background-color: #1ab394;
  color: #fff;
  border-radius: 3px;
  
  &:hover, &:focus {
    background-color: #18a689;
    color: #fff;
  }
}

.panel-footer {
    text-align: right;
    background-color: #fff;
}