That would then depend (in part) on the specific form being used. Is the foreign key part of the form? If so, then it would be a field in the form in the formset, and can be initialized. (But then it’s subject to being changed by the user.) If it’s not a field in the form and it can only be set within the POST handler, then you would need to save the individual forms with commit=False to get the new objects being created and update those objects before saving them.