Response for preflight has invalid HTTP status code 405

Looks like I have found solution. I just added second method:

[<OperationContract>]    
[<WebInvoke(UriTemplate = "PostTest", 
        RequestFormat= WebMessageFormat.Json,   
        ResponseFormat = WebMessageFormat.Json, Method = "POST")>]         
abstract PostTest: obj: Test -> unit

[<OperationContract>]    
[<WebInvoke(UriTemplate = "PostTest", 
        RequestFormat= WebMessageFormat.Json,   
        ResponseFormat = WebMessageFormat.Json, Method = "OPTIONS")>]         
abstract PostTestOptions: unit -> unit

It is just empty methods that do nothing. I don't know the reason, but all is working.

I want to show what work for me.

First you must enable CORS on web.config (like Mihai sad):

<httpProtocol>
  <customHeaders>
    <add name="Access-Control-Allow-Origin" value="*" />
    <add name="Access-Control-Allow-Headers" value="Content-Type" />
    <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE,     OPTIONS" />
  </customHeaders>
</httpProtocol>

If you have some extra HEADER parameter, you must add it to Access-Control-Allow-Headers, like:

<add name="Access-Control-Allow-Headers" value="Content-Type, X-Your-Extra-Header-Key" />

And finally, to handle OPTIONS requests you must reply with empty response, adding in your application class:

protected void Application_BeginRequest()
{
    if (Request.Headers.AllKeys.Contains("Origin") && Request.HttpMethod == "OPTIONS")
    {
        Response.Flush();
    }
}

Although this already has an answer, but heres my solution. In the web config you have to remove the instruction to <remove name="OPTIONSVerbHandler" />

First add in the customHeaders

<httpProtocol>
  <!-- THESE HEADERS ARE IMPORTANT TO WORK WITH CORS -->
  <customHeaders>
    <add name="Access-Control-Allow-Origin" value="*" />
    <add name="Access-Control-Allow-Methods" value="POST, PUT, DELETE, GET, OPTIONS" />
    <add name="Access-Control-Allow-Headers" value="content-Type, accept, origin, X-Requested-With, X-Authentication, name" />
  </customHeaders>
</httpProtocol>

Then either comment out or delete the instruction to remove the OPTIONSverbHandler

<handlers>
  <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
  <!-- <remove name="OPTIONSVerbHandler" /> -->
  <remove name="TRACEVerbHandler" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
</handlers>