So those records I see in your subform are locked against editing?
Okay, code can be like:
DoCmd.OpenForm "Post_Billing_Form", , , , acFormAdd, , Me.CustomerID
The trick will be figuring out what event to put that line in.
Then code behind the popup form.
Code:
Private Sub Form_Load()
If Me.NewRecord Then
Me.CustomerID = Me.OpenArgs
End If
End Sub
I am only guessing about the names for the primary and foreign key fields that link these tables. Adjust as needed.
Advise not to use spaces nor punctuation/special characters (underscore only exception) in naming convention. Better would be AccountNum or AccountNo or AcctNum.