A user's login key allows him to edit his personal form as well as to connect to the Signitic desktop application.
To copy the login key, go to the Users menu.
Then select the desired user.
In the Settings tab, you will find the Login Key.
Just click on the Copy button: a confirmation message will appear at the top right of your screen.
You can also send this key to a predefined user population using the method described in this article.