How to use select values for form action

I am trying to pass the select values for form action URL for wordpress,

    $args_shortcode = shortcode_atts(array(
    
        'category_name' => ''
    
    ), $attr);
    $condition = "";
    if(isset($_POST['Collateral_services'])){
      $condition = "/?Collateral_services=".$_POST['Collateral_services'];
    }
    $action_url = site_url() . "/category/" . $args_shortcode['category_name'] . $condition;

As you can see, the action url will redirect user to a different page which is getting fetched from shortcode attribute, so $_POST['Collateral_services'] is not fetching any data because it needs to be on search result page,

But the action URL needs to be based on selected option of the form, so I am not getting how can I change values in url after "/category/" using javascript or jQuery ?

form:

<form method="POST" action="<?php echo $action_url; ?>">

      <div class="form-group">
        <div class="col-xs-12 col-sm-12">
          <select name="Collateral_services" class="custom-select-input">
          <option value="" disabled selected>All Services</option>
            <?php
            foreach ($terms_ser as $term_ser) {
            ?>
              <option value="<?php echo $term_ser->slug; ?>"><?php echo $term_ser->name; ?></option>
            <?php } ?>
          </select>
        </div>
      </div>
<div class="custom-search-button-align">
        <input type="submit" value="Filter" class="custom-search">
      </div>
    </form>