Unfortunately, it might not be possible since as soon as the user moves to
the Subform (prior to entering anything in the Subform), the Record in the
main Form is updated into the Table. This is known as the AutoSave feature
of the Form/Subform combination.
Note that the Form/Subform combination is designed mainly for Tables in
One-to-Many relationship and the AutoSave feature is designed to enforce the
Referential Integrity of this relationship (the Parent / "One" Record *must
exist* before the Child / "Many" Records and it is possible to have Parents
*without* Children).
There is no way you can disable this AutoSave feature. There are
work-arounds but they are all messy and involve a fair bit of VBA coding.