If you need to add fields to the block version, you can use the Additional Fields API

This information is not clearly visible on the Woocommerce docs. It took me some while to understand that with the checkout block new API has been introduced. It would be VERY helpful to update misleading information on the blog with a hint to the new API. This blog post needs a warning: https://woocommerce.com/posts/customize-checkout-fields-woocommerce/

You're on the right track! We've just recently added the ability to display hide/show additional fields based on other fields. You can see how to do this in Conditional visibility and validation via JSON Schema docs.

Another question is how I can pass this the tax number to external app via API?

You can get the value of this field from the checkout data store with something like wp.data.select('wc/store/checkout').getAdditionalFields(). This will give you some key => value pairs of the name => value of the additional fields you registered. You can then either fetch to an external API directly from client, or build a custom endpoint on the server and do it there.

I notice also the issue that it is enought that Customer enter to the checkout page and I recive the order. There is not neccessary to enter any data and click the button.

I'm not quite sure why that's happening without seeing more code. Seems you must be calling the place order handler programmatically from your extension maybe?

I can't have the field for NIP number (namespace/gov-id) visible, I use following code:

`/**

• Dodaj pole NIP
  */

add_action('woocommerce_init', function() {
woocommerce_register_additional_checkout_field([
'id' => 'namespace/czy-nip',
'label' => 'Zaznacz, jeśli chcesz otrzymać fakturę VAT.',
'location' => 'contact',
'type' => 'checkbox',
]);
});

add_action('woocommerce_init', function() {
woocommerce_register_additional_checkout_field([
'id' => 'namespace/gov-id',
'label' => 'NIP',
'location' => 'contact',
'required' => [
"type" => "object",
"properties" => [
"additional_fields" => [
"properties" => [
"namespace/czy-nip" => [
"const" => "Yes"
]
]
]
]
],
'hidden' => [
"type" => "object",
"properties" => [
"additional_fields" => [
"properties" => [
"namespace/czy-nip" => [
"const" => "No"
]
]
]
]
],
'attributes' => [
'autocomplete' => 'government-id',
'pattern' => '[0-9]{10}', // 10 cyfr
'title' => 'Proszę wpisać NIP',
],
]);
});`

I understand you're looking for feedback here, but simply pasting (unformatted) code without much else is not the best way. I encourage you to look at the resources I mentioned, and if you're still struggling with the implementation, I suggest you head to WooCommerce Slack and ask in there. WooCommerce developers as well as community members hang out there and is a much better suited place to get your answers.