Require https on a web service in asp.net 4.0

Question

I have a web service that works only when the URL is http, but not https.

posting to https://mywebsite.com/api/webservice returns 404
posting to http://mywebsite.com/api/webservice connects.

connecting in the browser to https://mywebsite.com/login works

How can I enable https for this service?

    [OperationContract]
    [WebInvoke(Method = "POST", BodyStyle = WebMessageBodyStyle.Bare, UriTemplate = "/webservice", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json)]
    [Description("Post data to the webservice")]
    DataOut WebService(DataIn mp);