Add a column in dfs within a list on a certain position and use data from other df to fill it [closed]

I want to add a new column (Scannumber_1m) in dfs within a list (=dfs_list). Another df (single_df) contains values in the column row_start. I want now to take the value from df$row_start, go within dfs in list to the column Scannumber (continious numbers starting at 1) and when value of Scannumber == df$row_start I want to start counting in the new column (Scannumber_1m) to start counting from 1 to 500 the following rows. The other rows should contain 0.

I tried already, but only could fill in 0 in the new_column

new_list <- lapply(dfs_list, function(df) {
  df <- df %>% 
    add_column(Scannumber_1m = 0, .after = "Scannumber") 
  return(df)
})

Like that the new column is there already, but I don’t know how to start at certain row with the value from single_df$row_start with counting. names(dfs_list) == single_df$data.