A way to support the ‘OPTIONS’ pre-flight request in .net WebAPI is to by-pass this request with a default response and returning the required Headers.

Add the below code in your Global.asax.cs file under the Application_BeginRequest method:

protected void Application_BeginRequest()
{
	if (Request.HttpMethod == "OPTIONS")
	{
		HttpContext.Current.Response.AddHeader("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE");
		HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Content-Type, Accept, expiry, id, token, token-type");
		HttpContext.Current.Response.AddHeader("Access-Control-Allow‌​-Credentials", "true");
		HttpContext.Current.Response.AddHeader("Access-Control-Max-Age", "1728000");
		HttpContext.Current.Response.End();
	}
}

So when you call the WebAPI with Javscript fetch/axios, the pre-flight request sent before the actual request would get the 200 status code.

The example above shows some sample custom headers and allowed methods that will be returned in Response Headers.