You do not get an authentication key from Steam.
Now, if you wanted to install Blockland to a portable storage, Steam does have an option for that.
Click Steam > Settings > Downloads > Steam Library Folders > Add Library Folder
Then, backup any of your Blockland settings, saves, and add-ons. (this would be your add-ons, config, and saves folders), then reinstall Blockland on Steam, but this time it will give you the option to install it to another library folder.
Now, to play Blockland on another computer, you will also have to install Steam on that, log into it, then add the library folder on that computer, then you should be able to play the same Blockland copy just fine.