How do I connect a custom script to the form?

You can assign your custom script as a form submissions receiver in the Settings panel of the block containing the form.

1. Open the Content panel of the block containing the form.
2. Double click Form Data Receiver. https://www.youtube.com/watch?v=D3T4DYZeeGA
3. Select "Own script for receive data".
4. Enter the link to your script in the field that appears.

Data is delivered via POST.