MyDB tables are referenced in queries with a "mydb://" prefix on the tablename. For example,
result = queryClient.query('select * from mydb://galaxies', out='mydb://copy_of_galaxies')
where the 'galaxies' table is something you've previously created in your MyDB and you are saving the result to another MyDB table called 'copy_of_galaxies'. The same naming syntax applies when creating MyDB tables as the output from other queries from catalogs available in Data Lab.
Hope this helps, let us know if you continue to have questions.