How to Build Mutlipart Http Request with Serialized XML and PDF Attachment?

I am attempting to build an Http Request for a Post call to an API. The API documentation states I should send a content type of:

multipart/mixed;type="text/xml";boundary="MIME-Boundary"

It provides this example (redacted for length):

--MIME-Boundary
Content-Type: Application/XML
Content-Location: RN-Service-Content
Content-ID: pidxv10_invoice_xml

[xml content redacted]

--MIME-Boundary
Content-Type: application/pdf
Content-Transfer-Encoding: base64
Content-ID: Attachment1

[pdf base 64 content redacted]

--MIME-Boundary--

I stumbled upon the class MultiPartFormDataContent. I feel like this has gotten me close but I don't know if this is correct. If it is the class I should use I'm uncertain how to use it to match the provided example. I'm unit testing this code:

        public HttpRequestMessage BuildRequest(Models.InvoiceDTO invoiceDto, string pdfPath)
        {
            Invoice openInvoice = invoiceDto.MapOpenInvoiceData();
            string serializedInvoice = openInvoice.Serialize();

            string base64PDF = ConvertPdfFile(pdfPath);

            MultipartFormDataContent formData = BuildContent(serializedInvoice, base64PDF);

            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, apiSettings.EndpointURL)
            {
                Content = formData
            };

            return request;
        }

        public string ConvertPdfFile(string filePath)
        {
            Byte[] bytes = File.ReadAllBytes(filePath);
            return Convert.ToBase64String(bytes);
        }

        public MultipartFormDataContent BuildContent(string serializedInvoice, string base64PDF)
        {
            MultipartFormDataContent formData = new MultipartFormDataContent();
            formData.Headers.Add("Content-ID", "Attachment1");
            formData.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/pdf");
            
            formData.Add(new StringContent(base64PDF));
            formData.Add(new StringContent(serializedInvoice, Encoding.UTF8, "application/pidx.v10+xml"));

            return formData;
        }

For the sake of visualizing what I have I am executing request.Content.ReadAsStringAsync().Result in a test harness. When I do that I get:

--df45d3bb-617f-44c3-8aa4-b0f1411571bc
Content-Type: text/plain; charset=utf-8
Content-Disposition: form-data

[pdf content redacted]
--df45d3bb-617f-44c3-8aa4-b0f1411571bc
Content-Type: application/pidx.v10+xml; charset=utf-8
Content-Disposition: form-data

[xml content redacted]
--df45d3bb-617f-44c3-8aa4-b0f1411571bc--

I have a GUID where the example has the text --MIME Boundary. Is "--MIME Boundary" supposed to be in my content or is this MIME Boundary an example placeholder?

For both the PDF and XML content, how do I set their respective properties for Content ID, Content Location, Content Transfer Encoding?