I've done that using a hidden form. It has a timer event that checks the
value in a field in a Back End table. The table also has a field for the
message. If the first field's value is 0 nothing happens. If I update that
field to 1 the hidden form shows itself. It has the message field.
If you would like to see a sample, join my Yahoo group (in my signature) and
go to the files>Utilities and Add-ins folder. Download ShowUsers. It
contains everything including a admin form that shows who is in the
database, a button to lock out new users, a button to launch the
announcement form.