Could you give us more details on your problem? Does your problem appear when trying to add or save a new pair username/password in the FS-Key product?
What are the Operating system and the product version you are using?
on iPhones you need to copy and paste your username and password, there is unfortunately no automatic login functionality.
If you go to the card view you can copy for example your username or password by clicking the purple icons on the left of the text, the person shape or the box with the three dots inside it.
If you click the globe, this will open the site in your browser and F-Secure KEY will remain in the background. You can get back to KEY by double clicking the home button and finding it from the app list.
If you are still having trouble, please describe exactly which button press in which screen causes KEY to shut down.