A military general wishes to deliver important instructions to an allied leader who is a long distance away. To safeguard the message from being read by enemy spies during delivery, the general employs the services of a wizard. The wizard is able to cast illusory script, which provides a desirable level of security.
However, the wizard has never met or seen the intended recipient, although the general has. The general can give the wizard the recipient's name and description.
The description of the illusory script spell says:
To you and any creature you designate when you cast the spell, the writing appears normal.
Can the wizard designate a specific creature they have not met as being able to read their illusory script? How much does the wizard need to know about a creature to be able to designate them?
For context, I am the GM. I am doing a plausibility check on this plan. In the event of ambiguity, I can make an appropriate ruling.