JSFiddle - React, Tailwind, and code Playground

HTML

<select id="options">
<option value="">Choose...</option>                                                                       <option value="3370" >Tight Buffered </option>                                                                <option value="3371" >Loose Tube 900um </option>
<option value="3375" >Loose Tube 2mm </option>                                                 <option value="3372" >Break-Out 2mm </option>
<option value="3373" >Micro Cable 2mm </option>                        <option value="3374" >Nano Cable 900um </option>                                                <option value="3376" >Steel Tape Armoured 900um </option>
</select>
<select id="options2">
  <option value="">Choose...</option>
  <option value="3423" >12 MC</option>
  <option value="3424" >24 MC</option>
  <option value="3425" >48 MC</option>
</select>
<br />

<span class="display_text_here"></span><span class="display_text_here2"></span>
<br />

<input type="text" id="sku">

JavaScript

var sku_part1, sku_part2 = '';
$("#options").change(function(){

    switch($(this).val()){
        case "3370":
             sku_part1 = 'TB';
        break;
        case "3371":
             sku_part1 = 'LT';
        break;
        case "3375":
            sku_part1 = 'LTR';
        break;
        case "3372":
             sku_part1 = 'BO';
        break;
        case "3373":
             sku_part1 = 'MC';
        break;
        case "3374":
             sku_part1 = 'NC';
        break;
        case "3376":
              sku_part1 = 'STA';
        break; 
    }
    $('#sku').val(sku_part1);
});
$("#options2").change(function(){
    switch($(this).val()){
        case "3423":
              sku_part2 = '12';
        break;
        case "3424":
             sku_part2 ='24';
        break;
        case "3425":
             sku_part2 ='48';
        break; 
    }
    $('#sku').val(sku_part1+sku_part2);
});