code: def test_custom_monthly_seasonality(self, backend):
      m = Prophet(stan_backend=backend)
      m.add_seasonality(
          name="monthly", period=30, fourier_order=5, prior_scale=2.0
      )
      assert m.seasonalities["monthly"] == {
          "period": 30,
          "fourier_order": 5,
          "prior_scale": 2.0,
          "mode": "additive",
          "condition_name": None,
      }
  
imports: import numpy
  import pandas
  import pytest
  from prophet import Prophet
  from prophet.utilities import warm_start_params
label_desc: Use image node_id 1 for calling the TestProphetCustomSeasonalComponent obj's underlying member method code with example usage: obj.test_custom_monthly_seasonality(backend) without return types