you would need to add URL Variables for each form element, including the submit button to the bindings panel.
go to the bindings panel, click the plus button and select "URL Variable"
Use the form element name for the URL Variable name, repeat this for each form element you wish to validate, including the submit button.
NOTE: PHP is case sensitive. when adding the URL Variable bindings make sure to use the same case as the form elements names.
Once the URL Variables are created, you can use them as the Server Variables in the Server Validation behavior.
for the trigger, click the lightning bolt and select the submit button URL Variable binding