How can I use an Action parameter to set a property value to null?
By default, a modify rule will prefill a value to the previous property value. If you remove that prefill and set the parameter as required, it will null the property unless the user enters a value. If you remove the prefill and make the property hidden, it will null the property without displaying the parameter to end users.
In other words, do the reverse of what this tooltip is warning of
