Dear Sally,

Somehow, i have failed to get your question off my mind. Having thought through again, its much safer in your database to have this primary key field (TXO-061510-0065) to autofill itself rather than the user to fill it. This will prevent nulls and duplicates.
First, you would need to have the last four digits of the TXO-061510-0065 to be an autonumber.

Then you will put an After Update event in the first field on the form where you will enter data. Why there?, because when this field gets "dirty" that is what forces the autonumber to appear (Lets call this field CustomerName),
Suppose your autonumber field is called ID.

You will then write the code below in the CustomerName After update event

Private Sub CustomerName_AfterUpdate()

SerialNumber = "TXO-" & Format(Date, "mmddyy") & "-" & Format(ID, "0000")
    
End Sub

Thats not all

To lock the SerialNumber field, first you would set Tab Stop = NO. Then write the following code in the on got focus event

Private Sub SerialNumber_GotFocus()

    'Block any possibility to edit the field
    If Not IsNull(SerialNumber) Then
    SerialNumber.Locked = True
    ElseIf IsNull(SerialNumber) Then
    SerialNumber.Locked = False
    End If
End Sub

I am pretty sure this should work perfect.

Best regards,

MICAH