Do I have to write some code in .htaccess?

If the .htaccess file is not writable, in order for Hide My Wp Ghost to work you need to write the code you receive in the Hide My Wp Ghost > Permalinks after you change the settings.

If you don’t have server access you can install a plugin that will help you with this:

You can remove the wp-htaccess-control plugin after you’re done with the setup.

Is this plugin working if I don’t have custom permalinks on my site?

No. You need to have custom permalinks set on in Settings > Permalinks.

You will get a notification in the Settings page if something is not setup right.

Is this plugin working on WP Multisite?

Yes, this feature is available for Hide My WP Ghost version.
The Ghost version also works with Apache, Nginx, IIS and LiteSpeed servers.

You can see all the features here: https://wpplugins.tips/hide_my_wordpress/