Creating a new record in form mode

We use the main table (authors) and we have a linked field to another table (books).
If we don’t have an appropriate option (book) in the table we can create a new record of the book from the field in the author table. But, strangely and surprisingly, when we try to do the same thing through form, it is not possible. There is no option for creating a record in form mode (the plus “+” sign doesn’t exist).
This feature with control in permission for that (not in all fields you want to allow that kind of adding) is very important for our work.

Questions about that feature asked in the past, so you can see that this is an existing issue:

• Filling form - create a new record in linked table
• Unable to Add NEW Record on a Junction Table Form Field
• Can I put an "add a new record button" into a form to populate a row in my Customer table

Many thanks!