Woocommerce Checkout - Add conditional field required if one field is filled

Question

I want to add a conditional requirement on the Checkout page. Because a lot of people fill the Name of Company without they are a company. And I want to add to the "billing_company" field a conditional where if the customer fill the "billing_company" field need to be fill the "woocommerce_eu_vat_number" field too. The problem is those two fields are optionals. But I want to do them as required if billing_company /billing_vat_number is filled. Is it possible?

Field Codes in HTML:

<p class="form-row form-row-wide" id="billing_company_field" data-priority="30">
   <label for="billing_company" class="">Company name&nbsp;<span class="optional">(optional)</span></label>
   <span class="woocommerce-input-wrapper czr-focus">
      <input type="text" class="input-text czr-focusable" name="billing_company" id="billing_company" placeholder="" value="" autocomplete="organization">
   </span>
</p>

<p class="form-row form-row-wide" id="woocommerce_eu_vat_number_field" data-priority="120">
   <label for="woocommerce_eu_vat_number" class="">VAT number (companies)&nbsp;<span class="optional">(optional)</span></label>
   <span class="woocommerce-input-wrapper czr-focus">
      <input type="text" class="input-text czr-focusable" name="billing_vat_number" id="woocommerce_eu_vat_number" placeholder="" value="">
   </span>
</p>

I want a code something like this. Sorry because I can understand more or less php code but I don't know how to create it from zero. T_T

<?php

function custom_required_vat_billing_fields() {

if( !empty ('billing_company' ) {

        set ($fields['billing']['billing_vat_number'] = class [REQUIRED=true] );)

} if not {

    wc_add_notice(__('Please enter a VAT number if you are a company'), 'error');
}

};

    return $fields;

}
add_filter('woocommerce_checkout_fields','custom_required_vat_billing_fields');


?>

Answer 1

If checkbox is checked, then field company ID is required. Write code below to your child themes functions.php file and change ID of elements: wi_as_company is ID of checkbox and billing_company_wi_id is ID of required field if checkbox is checked

add_action( 'woocommerce_checkout_process', 'afm_validation' ); 
function afm_validation() {     
    if ( isset($_POST['wi_as_company']) && isset($_POST['billing_company_wi_id']) && empty($_POST['billing_company_wi_id']) ) {         
        wc_add_notice( __("Please fill company ID"), "error" );     
    }
 }