I could reproduce the situation with both SSMS 18.7 and 18.8. After I clicked the Script tab to generate the script on the Job Properties window, I could not do anything on the same window except pressing the Cancel button. Otherwise I would get the error message "The Job '_TestJob' does not exist on the server. (Microsoft.SqlServer.Smo)" even if I pressed the OK button. I am not sure if this is on purpose, but I could not find any reason to prevent the users from doing that.
I did not find the way to script the newly created custom categories since there is no column to store the date and time in the system table [msdb].[dbo].[syscategories], but the following script can list the counts of the jobs under the categories:
USE [msdb]; GO SELECT c.[category_id], c.[name], ISNULL(j.[JobCount], 0) AS job_count FROM [dbo].[syscategories] AS c LEFT JOIN ( SELECT [category_id], COUNT(*) AS JobCount FROM [dbo].[sysjobs] GROUP BY [category_id] ) AS j ON j.[category_id] = c.[category_id] WHERE c.[category_class] = 1 -- Job AND c.[category_type] = 1 -- Local GO