<select class="custom-select custom-select-lg mb-3">
  <option selected>Open this select menu</option>
  <option value="one">One</option>
  <option value="two">Two</option>
  <option value="three">Three</option>
</select>

<input type="text" name="catlist[]" value="1">
$('.custom-select').change(function (e) {
  $('input[name="catlist[]"]').val(this.value ==='two' ? 10 : 1);
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js