Retrofit: How to send a POST request with constant fields?

I want to sent a simple POST request with one actual parameter:

@POST("/token")
@FormUrlEncoded
void extendSession(@Field("refresh_token")final String refreshToken);

But this request should also send some constant values requested by the server such as client_id, client_secret and grant_type which are constant and should not be part of the application API.

What is the best way to do this?