This sounds like an odd arrangement between a pharmacy and a payer. If this payer is going to require drugs to be billed on a medical claim form, then you're going to need a diagnosis from the physician. This could potentially be problematic since physicians, as far as I know, don't normally include that information when prescribing a drug from a pharmacy. So you may need to set up a process by which you get this information, similar to the way a lab or diagnostic imaging center will require a diagnosis with their orders for a test. I'd caution against having the physician submit an ICD-10 code to you because physicians are notoriously poor coders and if you don't have access to their medical records, then you don't have a way to validate and/or correct their coding when needed. It would be best to have the physician give you a statement of the diagnosis along with the prescription and assign you ICD-10 code based on that.
This is not a normal way for pharmacies to bill so you're probably going to have to wait and see how it goes and troubleshoot the billing process as it evolves.