How to create custom field definitions with Sendgrid (v3) in C#?

Question

According to the V3 Sendgrid documentation, I can create custom definitions using their API: https://docs.sendgrid.com/api-reference/custom-fields/create-custom-field-definition

My code is as follows:

        public static async void addCustomDefinition(SendGridClient client)
        {
                var data = "{ 'name': 'test_name', 'field_type': 'Text' }";

                var response = await client.RequestAsync(
                    method: SendGridClient.Method.POST,
                    urlPath: "marketing/field_definitions", //"contactdb /custom_fields",
                    requestBody: data
                );

                Console.WriteLine(response.StatusCode);
                Console.WriteLine(response.Body.ReadAsStringAsync().Result);
                Console.WriteLine(response.Headers.ToString());

        }

However, I got the following output error message:

BadRequest {"errors":[{"field":"body","message":"invalid character '\'' looking for beginning of object key string"}]}

What am I doing incorrectly?

PS: I changed the urlPath parameter:

...from online guide: urlPath: "v3/marketing/field_definitions,

to this version: urlPath: "marketing/field_definitions",

this newer version works for me on other commands such as getting field definitions and creating contact list