You can post date medications/prescriptions simply by changing the Start Date on the directions pop-up screen.
If this screen does not pop-up when you search and select the medication, simply click on the "Edit" button on the Sig line to bring it up.
On this screen you can clear out the start date and add the future date.
If you need to add more than one prescription covering a longer period, it may be helpful to use the "Copy" button on the first one.
Then you can post date the second one to extend the period.
Just remember that the dates cannot overlap. For example, if one is for 3/1/2023 to 3/31/2023 then the next one must start 4/1/2023.
