If your form uses application/x-www-form-urlencoded, you can put the form controls as action parameters, like digitalpacman mentioned. You can also get them from the Request object:

var qs = this.Request.RequestUri.ParseQueryString();
string name = qs["firstname"];

If your form uses multipart/form-data, it's somewhat more complicated. See http://www.asp.net/web-api/overview/formats-and-model-binding/html-forms-and-multipart-mime