How to create custom loss function in catboost?

Question

I want to optimize SMAPE in my catboost model. When I pass the parameter loss_function = 'SMAPE' I get an error.

CatBoostError: Invalid loss_function='SMAPE': for regressor use RMSE, MultiRMSE, SurvivalAft, MAE, Quantile, LogLinQuantile, Poisson, MAPE, Lq or custom objective object

How can I create a custom loss function and use it in the model?

I don't understand what object needs to be passed to loss_function