/form [form_id [send | reject | [set | add] fieldname [value]]]

(lua command)
Without arguments prints form list.
With bare form id prints info on that form.
When setting multivalue field, new values are added, not replacing previous.
Without value unsets field, multivalue fields lose all of their values.
Add command allows to append field to form end.
Note, that form-specific default type will be used.