Sending hidden fields to integration

To send a hidden data to your integration, press the submit button to open its settings first:

Next, open the On Submit Actions list and find the relevant Update Custom Field action provided by the integration you connected: 

While setting up the action, you can specify the name for the hidden data and value, by typing in a text manually or by choosing one of the available dynamic values. To add the dynamic value, press the figure bracket icon on the right: