How do I set a form field using a passed URL parameter

If you are looking to automatically fill out a form field based on a URL parameter, you can use the dynamic term

%URL_PARAMS.<name of parameter>%

Thus if I wanted to set a field based on the value passed with a ULR parameter 'custom_status', 

https://www.acme.com/?custom_status=good

and the page  https://www.acme.com/ has an ActiveDEMAND form that you want a field set based on the parameter, for that field you would set the default value of the form field using the dynamic term:

The contact that fills out the form, the form element will render 'good' as the default value.