When configuring a List Block and the Edit record feature, there is the option to: Sync with Source: Allow adding new options. However new options are only possible when the source is a dropdown field directly in the table being accessed. If the source is a linked record field, and the original field is in another table, it is not possible to add a new record. Ideally, this would be possible. For now, it should be clear that linked records cannot be added this way.
I think this is the same issue I’m running into. Trying to use hidden field on form or add record to create a new record and automatically link the record to the related table. There’s a demo of this being done with Airtable, but I can’t get it to work with SmartSuite dataset. 