How to add_regressor when forecasting multiple timeseries with Prophet using R?

Question

I am using Prophet for a multi-channel leads forecast. I've been able to make the prediction using the method described by @RLave in this post Prophet Forecasting using R for multiple items. I would like to add regressors to my forecast. Below is the example given by @RLave along with an example of how I have tried to add regressor and the results. How do I make add_regressor work with my leads list?

# also contains the purrr package
library(tidyverse) 
set.seed(123)

tb1 <- tibble(
  ds = seq(as.Date("2018-01-01"), as.Date("2018-12-31"), by = "day"),
  y = sample(365)
  regressor = rnorm(365, mean = 0, sd = 1)
)

tb2 <- tibble(
  ds = seq(as.Date("2018-01-01"), as.Date("2018-12-31"), by = "day"),
  y = sample(365)
  regressor = rnorm(365, mean = 0, sd = 1)
)

# two separate time series
ts_list <- list(tb1, tb2) 

Build and prediction:

library(prophet)

# prophet call
m_list <- map(ts_list, prophet) 

# makes future obs
future_list <- map(m_list, make_future_dataframe, periods = 40) 

# map2 because we have two inputs
forecast_list <- map2(m_list, future_list, predict) 

Attempt to Add Regressor

m <- prophet()
m <- add_regressor(m, "regressor")

# prophet call
m_list2 <- map2(m, ts_list, fit.prophet ) 

Error: Mapped vectors must have consistent lengths:
.x has length 31
.y has length 14