What do I use for "recipient"?

I’m getting ready to send my first notification with Courier and it wants a recipient. What should I use for this value?

A recipient_id is a unique identifier for a recipient. This can be any string of your choosing. Just make sure to choose a value that doesn’t change. This prevents duplicate entries in the event that a recipient changes their email or some other identifying value. Typically, user ID values from your system make for good recipient_id’s.