How can I autoamtically assign a drawer?

If you only use a single drawer, and you don't want to track opening and closing of a drawer, you can use Simple Drawer setup. On the Settings tab, under Register, make sure you only have 1 active drawer, and select the "use simple cash drawer" check box.

This will automatically assign this drawer to all tickets, and keep the drawer open even after applying the Z process. You do not have to select the drawer when logging in, so you will want to use this option is you have set to automatically login as well.

The software by default, lets you manage as many drawers as you need. For example, if you are running 2 computers at the front desk for check out, you can track sales at each counter separately, and manage the money in each drawer.
However, if you only run 1 computer at the front desk, or only use 1 cash drawer, you can use the Simple Cash Drawer process, and the system will always assume you want to use the only open drawer. To enable this process, go to Settings, select the Register tab, and you will see a check box at the bottom right, that says "use simple cash drawer process." Select the check box, and it will always assume you have only 1 drawer.