To log in to the WMS App with your user, you need a four-digit PIN, which you can set up during the setup of the WMS App. Alternatively, you can set or change the PIN in your Shopware Admin.
You can access the user settings for the admin account at the bottom left via Your profile. In the General tab, in the Pickware Authentication section, you can set the app PIN.
You can find the settings for all other users under Settings → System → Users & Permissions. Click on the relevant user there and set the app PIN in the Pickware Authentication section. You can find more information on user roles for WMS here.