When you go to setup a WPS WiFi connection the printer should tell you the PIN to use via its LCD.
See page 20 of the Brother Network User's Guide; "Using the PIN method of Wi-Fi Protected Setup":
- Press "Menu".
- Navigate to and select "Network".
- Navigate to and select "WLAN".
- Navigate to and select "WPS w/PIN Code".
- When "network I/F switched to Wireless" is displayed, press "OK".
- The LCD will show an 8 digit PIN and the machine will begin to start searching for a WLAN access point/router for up to 5 minutes.