JSFiddle - React, Tailwind, and code Playground

HTML

<div>
                <label class="col-sm-2 mb-3">&nbsp;&nbsp;Ukuran</label>	
                <select name ="ukuran" id="ukuran" class="form-select-auto" onchange="pilihUkuran()" placeholder="Input ukurannya" method="post" required>
                  <option name="ukuran_1" id="ukuran_1" value="custom" method="post">Custom</option>        
                   <option name="ukuran_2" id="ukuran_2" value="10x10x5" selected>10x10x5 cm</option>
                  <option name="ukuran_3" id="ukuran_3" value="20x20x5">20x20x5 cm</option>
                  <option name="ukuran_4" id="ukuran_4" value="30x30x5">30x30x5 cm</option>
                </select>
              </div>

              <div id="customukuran1" style="visibility: hidden;">
                <select name="customukuran" id="customukuran" class="form-control" style="display:none;" method="get" >
                  <!-- MENDAPATKAN VARIABEL P x L x T -->
                  <input class="customp" id="custom_panjang" onkeyup="mult(this.value,document.getElementById('custom_lebar').value,document.getElementById('custom_tinggi').value);" name ="custom_panjang" type="number"  placeholder="Panjang (cm)" >

                  <input class="customl" id="custom_lebar" onkeyup="mult(document.getElementById('custom_panjang').value,this.value,document.getElementById('custom_tinggi').value);" name ="custom_lebar" type="number" placeholder="Lebar (cm)">

                  <input class="custom3" id="custom_tinggi" onkeyup="mult(document.getElementById('custom_panjang').value,document.getElementById('custom_lebar').value,this.value);" name ="custom_tinggi" type="number" placeholder="Tinggi (cm)">

              </select><br>
              </div>       
              
              <input class="mr-5 rupiah" name="rupiah" id="rupiah"  disabled style="color: black; font-weight: bolder;">

JavaScript

function pilihUkuran(){
                  var custom = document.getElementById("ukuran");
                  if (custom.value == "30x30x5"){
                    document.getElementById("customukuran1").style.visibility="hidden";               
                  }else if (custom.value == "10x10x5"){
                    document.getElementById("customukuran1").style.visibility="hidden";
                  }else if (custom.value == "20x20x5"){
                    document.getElementById("customukuran1").style.visibility="hidden";
                  }else
                    document.getElementById("customukuran1").style.visibility="visible";
                  }
               
    function mult(pj,lb,tg){
          var isipj=pj;
          var isilb=lb;
          var isitg=tg;
          var hasil=isipj*isilb*isitg;
      document.getElementById('rupiah').value = hasil;  
}