If you are on a trial Premium it doesn’t matter if you’ve purchased or not.
Right-clicking and encrypting a folder does not designate it as a Secured Folder. You have to do that from the AxCrypt ‘advanced’ window.
Once you’ve done that, you can encrypt files that appear in the folder unencrypted with a single click, or by signing out or exiting the app. They won’t be encrypted fully automatically, because it doesn’t work with many applications that create and save files.