Good question, but it is unfortunately not possible at the current time.
I suspect you want to send a scheduled downlink to a sensor at a specific time. There are other ways to do this but will need some additional setup. One way is to use NodeRED instead. They have a CRON plugin with schedules working similar to IoT Open. If you want IoT Open to save the time you could also create a trigger function in IoT Open that is controlled using the build in schedule function and then connect the trigger to a NodeRED flow where the MSG is added and then sent on the downlink function.
