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var request = new FooRequest
{
    Id = "123",
    AdditionalProperties =
    {
        ["extra1"] = new { value = 42 },
        ["extra2"] = DateTime.Now,
        ["extra3"] = 99
    }
};
var response = await client.Foo.CreateAsync(request);
Dictionary<string, JsonElement> jsonElements = response.AdditionalProperties.ToJsonElementDictionary();
Dictionary<string, int> extra1 = jsonElements["extra1"].Deserialize<Dictionary<string, int>>();
DateTime extra2 = jsonElements["extra2"].GetDateTime();
int extra3 = jsonElements["extra3"].GetInt32();