Access will not give this to you, so you would need to create a table where
you record who currently has the form open. You will need to append a record
there in the form's Open event, and remove it in the form's Close event. If
there is a record in the table, cancel the Open event to prevent another
user opening the form as well.
You will also need to provide some way to clean up in case a user crashes
with the form open. Otherwise your application would never let anyone into
the form again.
You have not told us your reasons for wanting to build the app this way, but
my guess is that there is a better way to solve whatever issue you are
attempting to avoid.