Correct syntax for creating an incident with sub custom object

Summary

What is the correct syntax for creating an incident that contains a primary sub-object?

Content

1. I created a custom object (lp_predio),
2. Associated it with a custom field in the Incident table (id_predio), making the Incident the parent table,
3. Created a custom widget to display its values in the /app/ask form and submitted everything to a custom controller.

All values are sent, but the custom object value, is not saved in his respective field in the Incident, only the values from primary and custom fields. I am always getting the Type mismatch: Must be a lp_predio; Incident.CustomFields.dtpinterno.id_predio