**# define the loss function**
def my_function(y_true, y_pred):
**# create scorer using sklearn**
from sklearn.metrics import make_scorer**
**my_own_scorer = make_scorer(my_function, needs_proba=True)
**catboost = create_model('catboost')
**# tune catboost using custom scorer
**tuned_catboost = tune_model(catboost, custom_scorer = my_own_scorer)