If you have a patient who needs a prescription for something you do not have in inventory, you can simply enter the details into a Treatment (just type in the text, the inventory auto-complete will go away once you've typed enough that nothing in inventory matches).
In the "Processing and Printing" section, you'll see an option to "Script out" the treatment. If you do not see that button, go to Clinic Settings --> Clinic Options and choose "Enable printing of prescription forms".
Once you click the "Script Out" button you'll be given a few options for how you want this prescription form to print, then you'll see a preview of the form.