Flurl \ RestSharp: `\r\n` is added to each parameter of the Multipart request

Question

I'm trying to write a call to the Freshdesk API using either Flurl or RestSharp library.

The API call I'm trying to write is Creating a Ticket: https://developers.freshdesk.com/api/#create_ticket

An example of cURL:

curl -v -u yourapikey:X -F "attachments[]=@/path/to/attachment1.ext" -F "attachments[]=@/path/to/attachment2.ext" -F "[email protected]" -F "subject=Ticket Title" -F "description=this is a sample ticket" -X POST 'https://domain.freshdesk.com/api/v2/tickets'

Note params are passed by -F (i.e. --form) flag.

With the following code calling Flurl:

var rs = await url.WithBasicAuth(_configuration.ApiKey, _configuration.Password)
                  .PostMultipartAsync(mp =>
                  {
                      mp.AddString("email", ticket.Email);
                      mp.AddString("subject", ticket.Subject);
                      mp.AddString("status", ticket.Status.ToString());
                      mp.AddString("priority", ticket.Priority.ToString());
                      mp.AddString("description", ticket.Description);
                  });

...Or the following code calling RestSharp:

var request = new RestRequest("tickets", Method.Post);
request.AlwaysMultipartFormData = true;
request.AddParameter("email", ticket.Email);
request.AddParameter("description", ticket.Description);
request.AddParameter("subject", ticket.Subject);
request.AddParameter("status", ticket.Status);
request.AddParameter("priority", ticket.Priority);

var rs = await client.ExecuteAsync(request);

...I'm receiving 400 BadRequest with the following information (in both cases):

{
    "description": "Validation failed",
    "errors": [
        {
            "code": "invalid_field",
            "field": "email\r\n",
            "message": "Unexpected/invalid field in request"
        },
        {
            "code": "invalid_field",
            "field": "subject\r\n",
            "message": "Unexpected/invalid field in request"
        },
        {
            "code": "invalid_field",
            "field": "status\r\n",
            "message": "Unexpected/invalid field in request"
        },
        {
            "code": "invalid_field",
            "field": "priority\r\n",
            "message": "Unexpected/invalid field in request"
        },
        {
            "code": "invalid_field",
            "field": "description\r\n",
            "message": "Unexpected/invalid field in request"
        }
    ]
}

Note that:

• Freshdesk API uses -F / --form as a way to pass parameters.
• I want to use Multipart for handling attachments later on.
• All data has proper values. If it wouldn't have a proper value, a validation error would come as a response.
• When sending the same values in Postman (via form-data), I'm receiving 201 Created.

Update 1

Postman screenshot (working example):

Update 2

I tried to use Tiny.RestClient which has cURL listener and it came with the following cURL generated:

curl -X POST [...] -d "--1c3acb07-77bd-494d-bc56-21290dcd5088
Content-Type: text/plain
Content-Disposition: form-data; name=email

[email protected]
[...]

Perhaps, Flurl and RestSharp uses the same way to proceed with parameters and thus, the \r\n in fields.