This one?

User Level Security

I think that's it Kevin. I realized just now that what I am trying to add to it is something in the code of the LogonBtn On Click event... Currently it checks for the last time a user's password was changed using this:

DateChaged = Nz(DLookup("DateChanged", "UserT", "Username=""" & TempVars("Username") & """"), #1/1/1990#)
    If Date - DateChaged > 90 Then
        MsgBox "Your password is more than 90 days old and must be changed now."
        DoCmd.OpenForm "PasswordChangeF"
    Else
        DoCmd.OpenForm "Main Menu"
    End If

I'd like to add in there somewhere for it to check if a new column I've added to the UserT ("FirstLogon") is null so that if it is, some other code can be run elsewhere.

Also, this is the code that runs in the passwordChangeF so I assume something like this would have to be added in somewhere:

Private Sub UpdateBtn_Click()

    Password2.SetFocus
    If IsNull(Password1) Then MsgBox ("Password 1 missing"): Exit Sub
    If IsNull(Password2) Then MsgBox ("Password 2 missing"): Exit Sub
    If Password1 <> Password2 Then MsgBox ("Passwords don't match!"): Exit Sub
    
    CurrentDb.Execute "UPDATE UserT " & _
        "SET Password=""" & Password1 & """, DateChanged=#" & Date & "# " & _
        "WHERE Username=""" & TempVars("Username") & """"
        MsgBox ("Password updated successfully!")
    
    DoCmd.OpenForm "Main Menu"
    DoCmd.Close acForm, Me.Name, acSaveYes

End Sub

Have you watched this video?
Force Change Password

Yes, thats where I got the code listed above.

I added this:

If IsNull("UserT!FirstLogon") Then
        UserT!FirstLogon = Date
    End If

but it does not work.

You need to look up the value of the field in the table. The code below assumes there is a record. You may need more if the user doesn't exist. I hard coded the UserID for simplicity and testing purposes.

Dim FirstLogon As String, UserID As Long
    
UserID = 1
FirstLogon = Nz(DLookup("FirstLogon", "UserT", "UserID=" & UserID), "")
    
If FirstLogon = "" Then
    CurrentDB.Execute "UPDATE UserT SET FirstLogon=#" & Date & "# WHERE UserID=" & UserID
End If

Kevin, you sir, are a GENIUS! Thank you so much!

You're welcome.

Daniel, Kevin Robertson is a true GENIUS