If I understand well, Subject is a reference and not a simple String attribute input in your Smart Action form.
It seems logic that Subject: subject.name does not work, as the the name of the subject is not what is expected here. Did you try: Subject: subject or maybe Subject: subject.id?
However, from what I can see here you’re just trying to re-create an edit form. Which already exists when you click on a record.
I need to add custom fields (that is, extra fields in addition to the ones that exist in the table). I am processing the value of these fields in the Smart Action’s routes.
I see that it is possible to override the routes of existing CRUD actions.
But Is it possible to have extra fields in the create/edit forms? If yes, how can I add it?
I need to add custom fields (that is, extra fields in addition to the ones that exist in the table)
You could consider creating smart fields and configure their visibility and behavior. Then you could see them in the default edit/create form of Forest Admin without having to create your own form (smart field can be hidden in table view). Still, I’m not totally sure it would fit your need, but you could give it a try