Start WinBox or use WebFig and log in to the router. Click on Wireless -> Security Profiles tab -> click on the + sign and create your security settings (what shall be the encryption type, what shall be the password, etc. – on the screenshot you see a suggested configuration), then click OK
Go back to the Interfaces tab, click on the + sign and choose VirtualAP
In the appearing window set the SSID, set the Master Interface to wlan1 (on dual-band devices usually this is the 2.4 GHz radio interface, wlan2 is the 5 GHz one) and set the Security Profile to the one you have just created, then click OK (please note the name of this newly created wireless interface).
Now click on Bridge -> Ports tab -> click on the + sign and select the newly created wireless interface and assign it to the bridge-local bridge, then click OK.
