var client = new RestClient("baseUrl");
var request = new RestRequest("resource").AddBody("body", "multipart/mixed; boundary=changeset");
await client.PostAsync(request);
Expected behavior

To be able to send a string body, with the content type "multipart/mixed; boundary=changeset".
Stack trace
System.FormatException: The format of value 'multipart/mixed; boundary=changeset' is invalid.
   at System.Net.Http.Headers.MediaTypeHeaderValue.CheckMediaTypeFormat(String mediaType, String parameterName)
   at System.Net.Http.Headers.MediaTypeHeaderValue..ctor(String mediaType)
   at System.Net.Http.StringContent..ctor(String content, Encoding encoding, String mediaType)
   at RestSharp.RequestContent.Serialize(BodyParameter body)
   at RestSharp.RequestContent.AddBody(Boolean hasPostParameters)
   at RestSharp.RequestContent.BuildContent()
   at RestSharp.RestClient.ExecuteInternal(RestRequest request, CancellationToken cancellationToken)
   at RestSharp.RestClient.ExecuteAsync(RestRequest request, CancellationToken cancellationToken)
   at RestSharp.RestClientExtensions.PostAsync(RestClient client, RestRequest request, CancellationToken cancellationToken)
          
You can use
var request = new RestRequest("resource") { FormBoundary = "changeset" }
     .AddHeader(KnownHeaders.ContentType, "multipart/mixed");
When RestSharp sees you are setting the content type header without the actual body, it will add an empty body.
You can use
var request = new RestRequest("resource") { FormBoundary = "changeset" }
     .AddHeader(KnownHeaders.ContentType, "multipart/mixed");
When RestSharp sees you are setting the content type header without the actual body, it will add an empty body.
I tried this, along with AddStringBody("body", DataFormat.None) after the AddHeader, but the Content-Type was only set to multipart/mixed and not multipart/mixed; boundary=changeset
It would help if you specify how do you need your request to look like.
I am trying to use a batch request of an OData API, similar to the example here https://www.odata.org/documentation/odata-version-3-0/batch-processing/
The body is a string I have already built.

The Content-Type header needs to be multipart/mixed; boundary=changeset.
          
I got this code:
var request = new RestRequest("resource") { FormBoundary = "changeset", AlwaysMultipartFormData = true};
request.AddStringBody(json, ContentType.Json);
var response = await client.PostAsync(request);
and it produces the following result:

          
That works in regards to setting the Content-Type header correctly, thanks.
The next problem is, I need to format the request exactly like this:
Header:

Content-Type: multipart/mixed; boundary=changeset
Body:
--batch
Content-Type: multipart/mixed; boundary=changeset
--changeset
Content-Type: application/http 
Content-Transfer-Encoding: binary 
Content-ID: 1
PUT https://api.com HTTP/1.1
Content-Type: application/json
{ "foo": "bar1" }
--changeset
Content-Type: application/http 
Content-Transfer-Encoding: binary 
Content-ID: 2
PUT https://api.com HTTP/1.1
Content-Type: application/json
{ "foo": "bar2" }
--changeset
--batch--
The set of --changeset's are wrapped with a --batch / --batch--.

The same Content-Type as the header is repeated after the opening --batch.

Previously I created the entire body as a string, from the opening--batch to the closing --batch-- with all of the --changeset's in between.

Using the method above I have tried AddStringBody() and AddBody() but it always wraps my body within a --changeset.
Is this supported?