Using a custom ISO8601DateFormatter with JSONEncoder

Question

I have the following date formatter:

let formatter = ISO8601DateFormatter()
formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]

I want to pass that into a JSONEncoder, like this:

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .formatted(formatter)

but that produces an error as it only accepts custom DateFormatters not ISO8601DateFormatters.

How can I do this?