Do you have the default value of your ID field set? Normally in that kind of setup you would have like your CustomerID defaulted to Forms!CustomerF!CustomerID or whatever, then when you open the form it would pull in the default value and let you add new things.

Hi Don
I don't know what your record source is, but why not put the subform in the footer of the continuous form each with its own table and eliminate the button. Access will initially tell you that you can't and it will also change the continuous form to a single form but simply change it back to a continuous form. Make sure it is linked.
John

Hi Don,
An example to look at

John

Hi Don, I forgot that you were using a continuous form. Here is a new image
John

Thank you for all of your advice. I really appreciate it. It looks like my problem was not having the "=" in the default value. Richard's video on the Invoicing Template eventually led me to the answer. I was getting all kinds of errors. For example, the ID field on the last record (where you would add a new record) defaulted to the first record on the main form. Other times, I got the message indicating that the subform was not updateable.  This drove me crazy. Anyway, thank you again.