To connect to a client, you need to tap the bold font in the upper right corner, then tap "plus" symbol.
When the new window comes up:
Here you enter the username and password that you must have received from your manager / accountant.
Then, select your user from the pull-down menu and enter the password that should be in the same email from the manager / accountant:
Here you select the user you want to connect with and the password that is for that user.