​

let select = document.querySelector("div[data-uniqueid='myUniqueId'] select")

$(select).materialSelect();

​

select.onchange = function(){

  let displayEl = document.querySelector('div[data-uniqueid="myUniqueId"] input.select-dropdown');

  displayEl.value  = this.options[this.selectedIndex].getAttribute('data-display-value');

}

select.onchange();