The BSSID or MAC-Address is a unique, non-editable property of the WLAN-router or of the WLAN-device that is used to provide your own WLAN-network. It consists of hexadecimal pairs of numbers and mainly comes in formats like "08-00-20-ae-fd-7e" or "08:00:20:ae:fd:7e".

To get the BSSID of your WLAN-network please log into the configuration of your WLAN-device. It highly depends on your device and the vendor of it where you can find that property exactly. In most cases it is related to itms like "Extended Configuration" or "Wireless Network Parameters" or "Device Information".
Please Note: Your router may use two MAC-addresses, one for the local, wired network and one for the wireless network! Here only the wireless MAC-address is the right one, the other one can't be used. In most cases the correct MAC-address will be named as BSSID.
Below you can find helpful screenshots of some WLAN-routers: