fstdropdown initializes on page load, just add class "fstdropdown-select" to your select.
<select class='fstdropdown-select'> </select>
If you dont need search in dropdown,just add data-searchdisable="true" to select.
<select class='fstdropdown-select' data-searchdisable="true"> </select>
If you add select dynamically, add "fstdropdown-select" to it and call function setFstDropdown().
If you change your select options list, use document.querySelector("your select").fstdropdown.rebind() to update dropdown list.