I'd use a custom function for this - assuming the underlying table is called tblInvoices
:
Function GetBillingMonthOfLatestInvoice()
Const SQL = "SELECT TOP 1 Billing_Month FROM tblInvoices ORDER BY [Invoice_#] DESC"
Dim RS AS DAO.Recordset
Set RS = CurrentDb.OpenRecordset(SQL)
If RS.EOF Then
GetBillingMonthOfLatestInvoice = Null
Else
GetBillingMonthOfLatestInvoice = RS(0)
End If
End Function
Update
The above code can be generalised to return other related fields like so:
Function GetValueForLatestInvoice(FieldToLookUp As String)
Dim RS As DAO.Recordset, SQL As String
SQL = "SELECT TOP 1 " + FieldToLookUp + " FROM tblInvoices ORDER BY [Invoice_#] DESC"
Set RS = CurrentDb.OpenRecordset(SQL)
If RS.EOF Then
GetValueForLatestInvoice = Null
Else
GetValueForLatestInvoice = RS(0)
End If
End Function
To use, copy the code to a new standard module, then for each relevant text box on the form, set its Default Value property in the Properties window to something like this:
=GetValueForLatestInvoice("Billing_Month")
That would be for the text box holding the billing month value; for the one holding the billing year, you would use
=GetValueForLatestInvoice("Billing_Year")