Teddan you can't specify multiple values with a comma. Your example code currently inserts 2 lines in the config.php array, one has key url and your hard-coded URL, and the second line has no key and just a string value, so it's ignored by Flarum when reading the config file. The dynamic value needs to be assigned as the value for url. Luceos example in the post you link seems to use the correct syntax.
