Bert,
You can write that in the comment section on the New Crop screen. The question will be whether the pharmacist will hold that e-prescription in queue until said future date. My brother is a pharmacist and I will ask him if his company permits him to hold the e-scripts.
My experience has been that if the pharmacy does not have a particular medication in stock, say Adderall 10mg, then they cancel the e-script, and I have to e-prescribe it to another pharmacy (or have the patient come back and pick up a paper script).
For the ADHD meds, I have not been e-prescribing but printing out the scripts as normal because of pharmacy supply issues.