refit: Refit doesnt allow constant variable from another class ?

How to define a constant variable for path “index.php?route=mobile” ? I defined in another class a constant variable.

Config.cs

public const  String SERVICE_PATH = "/index.php?route=mobile";

APIService.cs

 [Post(Config.SERVICE_PATH+"/login")]
        Task<string> login([Body(BodySerializationMethod.UrlEncoded)] Dictionary<string, string> userInfo);

But refit throws an error like this :

System.NotImplementedException: Either this method has no Refit HTTP method attribute or you’ve used something other than a string literal for the ‘path’ argument.

Full part of my service interface :

public interface APIService
{
        [Post("/index.php?route=mobile/login")]
        Task<string> login([Body(BodySerializationMethod.UrlEncoded)] Dictionary<string, string> userInfo);

        [Post("/index.php?route=mobile/account/register")]
        Task<string> register([Body(BodySerializationMethod.UrlEncoded)] Dictionary<string, string> userInfo);

        [Get("/index.php?route=mobile/account/forgotPassword&email={email}")]
        Task<string> forgotPassword(String email);
}