Retrofit Post Parameter

You can also pass multiple field parameter: for example:

@FormUrlEncoded
@POST("/oauth/access_token")
void getToken(
    @FieldMap Map<String, String> params, 
    Callback<FacebookLoginUserResponse> callback
);

Try using this

public interface SafeUserApi {
 @FormUrlEncoded
    @POST("/api/userlogin")
    void getUserLogin(
            @Field("client_id") String id,
            @Field("client_secret") String secret,
            @Field("username") String uname,
            @Field("password") String password,
            Callback<LoginResult> cb
    );
}

Here parm1 is the POST parameter that you will be passing it to the server. This will solve your problem

in case if you are using PHP u can access the param1 using $uname= $_POST('username');

EDIT 1:

retrofit 2.0 version:

public interface SafeUserApi {
    @FormUrlEncoded
    @POST("/api/userlogin")
    Call<ResponseBody>  getUserLogin(
            @Field("client_id") String id,
            @Field("client_secret") String secret,
            @Field("username") String uname,
            @Field("password") String password
    );
}