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Single and Multi-select linked fields in forms should have a toggle in rules to allow users to add a record.
Cleanly and intuitively allowing form users to add new options which may become new trends. @ScottWorld kindly provided some work-arounds, but the ideal one would be built-in and mirror the UI of adding a record in grid view.
Add records in a link to another record field in a form
When adding a new record from a form how do I add a new record "on the fly"
HELP - Attendance log with add new record option in a form or interface
Also my boss was frustrated that it wasn't an option out of the box and we have a project on pause now because of it.
I think this is a feature that will be used for companies using AirTable for internal processes. In my case our employees encounter new types of customer types and customer issues and I want them to be able to cleanly and intuitively add new options which may become new trends.