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',
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.