How do I connect a custom script to the form?

1. Open Site settings → Forms → Webhook.
2. Add the address of your script.
3. Confirm or skip the step "Assign the service to all forms on the website?".
4. If If you skipped the previous step, go to the web page Editor, open the Content panel of the block containing the form and select Webhook as a Form data receiver service.
5. Publish all pages.

Data will be sent through POST to the address of the script you have specified in your settings.

Read more: http://help.tilda.cc/formswebhook