[w3schools]

Вариант раз https://jsfiddle.net/Politonius/27Lv010L/
Вариант два с удалением дефиса https://jsfiddle.net/Politonius/27Lv010L/1/

<form action="" method="post">
<input type="text" id="test">
</form>
 <div id="result"></div>

<script>
var input = document.getElementById('test');
var result = document.getElementById('result');

input.onkeyup =  input.oncopy = input.onpaste = input.oncut = (function() {
    return function() {
        var mytest = this.value.substr(0,4);
        var count = this.value.length;
        
        if(count > 4){
             if(mytest == "TUR-"){
                 result.innerHTML = this.value;
                 }else{
                 result.innerHTML = "TUR-" + this.value;
                 }
             }
   }
})();
</script>