How to enable record addition on a subform

[San_Tou]

I have a table tbl_function that self reference as well as referencing another table
I am struggling to add record using the subform, it seems that the ‘add’ is nowhere to be found (see photo)
I noticed that some other tables allow me to add record this way.
May I ask how to turn on such feature in a subform?
Your help is appreciated.

[Steve]

The Ref column of the child table must have Is A Part Of enabled.