Display selected values in multiselect

Hi! I'm using material design multiselect for submitting form. Since it seems to me 'selected' attribute is not working here, what would be the easiest method to iterate through the array of database values and change options to 'selected' accordingly.

{
key: [ "option1", "option3"]
}

<select class="mdb-select md-form" id="exampleID" name="key" multiple>
          <option value="" disabled selected><%= loopArray(user.key) %></option>
          <option id="option1" value="option1">option1</option><!–– selected ––>
          <option id="option2" value="option2">option2</option>
          <option id="option3" value="option3">option3</option><!–– selected ––>
          <option id="option4" value="option4">option4</option>
          <option value="Not specified">Not specified</option>
          <label class="mdb-main-label">Choose option</label>
        </select>
      <button type="button" class="btn-save btn btn-primary btn-sm">Save</button>

Thanks!