Impersonation takes place whenever your credentials are not enough to complete current operation. Impersonation in Windows can take different forms, must common are UAC (User Account Control) prompt and "Run as a different user" prompt.
In both cases you would need to click on More choices to be able to select Use SystoLOCK account tile.
Here you now key in your user name, your PIN and OTP. If you logged in previously using SystoLOCK on this workstation, there will be a tile with your user name pre-filled - you can also select that tile.