Add new regressor into Prediction dataframe

Question

In the df_train is column 'total' with amount and this is regressor for fitting.

How can I add regressor into future dataframe?

I need to take amount from df_train 12 months ago from future['ds'].

    model = prophet(interval_width=0.95,seasonality_mode='multiplicative',changepoint_prior_scale=0.05,seasonality_prior_scale=0.08,yearly_seasonality=True)   
    # Add the new future regressor
    model.add_regressor('total')
    model.fit(df_train)
    future=model.make_future_dataframe(periods=14, freq='MS')
    future['total'] = future['ds'].apply()????????????
    forecast = model.predict(future)

Help with code please.