Webservice called by email button

I am trying to develop a webservice that will be called by a button or link in an email. So the webservice call needs to be contained within one URL link.

I can get this working as a GET, but as a POST I am having trouble with the syntax.

I need to pass it a passenger ID as follows:

[creatio URL]/0/rest/UsrManifestPassengerFeedback/insertPassengerFeedback?passengerID=abcd

I am getting Method Not Allowed and I'm pretty sure it is just an issue with my URL syntax or something about the method definition, since as a GET it works fine.

 namespace Terrasoft.Configuration.UsrManifestPassengerFeedback
{
    using System;
    using System.ServiceModel;
    using System.ServiceModel.Web;
    using System.ServiceModel.Activation;
    using Terrasoft.Core;
    using Terrasoft.Web.Common;
    using Terrasoft.Core.Entities; 
 
    [ServiceContract]
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
    public class UsrManifestPassengerFeedback: BaseService
    {
        // Link to the UserConnection instance required to access the database.
        private SystemUserConnection _systemUserConnection;
        private SystemUserConnection SystemUserConnection {
            get {
                return _systemUserConnection ?? (_systemUserConnection = (SystemUserConnection)AppConnection.SystemUserConnection);
            }
        }
 
        // A method that returns the contact's ID by its name.
        [OperationContract]
		[WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.Wrapped,
        ResponseFormat = WebMessageFormat.Json)]
 
        public string insertPassengerFeedback(string passengerID){
 
 
 
			return "failure";
        }
 
    }
}