How to add HTML, CSS or Javascript?

Go to the blocks library → Other → T123 block "Embed HTML code". You can also type T123 in the blocks library search box.

The block supports HTML, JavaScript (inside the <script> tag) and CSS (inside the <style> tag). Unfortunately, you cannot add your PHP code to the page, but you can add it to your server and send data to it using Webhook.

Note that Tilda doesn't provide support for any issues related to the external code.