There are two ways:

1. In »Manage Cashy > Settings > Extra» you find an option »Show button ‹Open drawer on main screen»
2. If you wish it more discrete, you can let the option mentioned above deactivated. Taping the »00»-button on the numerical keyboard followed by any category-button will have the same effect and sends the open-impulse to the drawer

Using one of this two methods will a) print a short receipt and it will be mentioned in the log