How to make a swift Codable struct that will decode and encode a MongoDB _id ObjectId() in JSON

I'm a very new developer(this is my first dev job) building a Swift/iOS application for creating/processing orders, and the server is sending me a ObjectID() object in JSON with every product I look up. After some research, it seems like this is a MongoDB object.

The gentleman coding my API routes wants me to grab that object for every product the server sends me, so I can send it back to the server with any orders that include that product. He says that will make it much easier for him to access the product when processing new orders.

So far, I've had no trouble decoding the JSON the server is sending me, because it's been in formats like String, Int, Float, etc., or just another JSON object that needs a new Codable struct full of more of the same.

When it comes to creating a Codable struct for an ObjectID, I don't know what keys/value types (properties/value types, whatever terminology you want to use) to tell it to expect. Or if this is even the correct way to go about it.

This is what I have right now:

import Foundation

struct ProductData: Codable {
    let _id : ObjectId
    let productId : String
    let description : String
    let ...
}

The ObjectId type appearing above is a custom Codable struct that I haven't built yet, because I'm not sure how. I imagine it should look something like this:

import Foundation

struct ObjectId : Codable {
    let someVariableName : SomeCodableType
    let ...
}

I don't know what the variable name or the type would be. I understand that it has a timestamp and some other information inside of it, and I've read about it being represented as a string, but I get the feeling if I try something like let _id:String in my product Codable struct, it won't decode/encode the way I'm imagining.

I'm wondering how to build a "type" that will properly catch/decode the _id object that is being thrown at me. If there's a way to simply hold that data without decoding it, and just send it back when I need to later, that would also suit my purposes.

EDIT: After some experimentation, I found this raw JSON for the _id object:

"_id":{"$id":"58071f9d3f791f4f4f8b45ff"}

Dollar signs are not allowed in Swift variable/property names, so I'm unsure how to proceed in a way that satisfies both the incoming AND outgoing - I could make a custom key by manually initializing the JSON so it will properly work with Swift, but I'm unsure if there's a way to reverse that when encoding the same object back into JSON to send back to the server.

Catching the JSON as a Dictionary of [String:String] seemed to do the trick.

import Foundation

struct ProductData: Codable {
    let _id : [String:String]
    let productId : String
    let description : String
    let ...
}

However, the server is struggling to convert this back into an OrderId() object - I'm guessing that the translation from "$id":OrderId("someid") to "$id":"someid" is not what should happen.