In the end - the goal is to add a wireless printer (Brother MFC-J6510 DW) to the network and it asks me for the network key.
It is asking you for the WPS.
Read the info about setting up wps printer in 6.2 utility.
Select the pin method and you set a pin.. or the pin is fixed by the application for setup of the brother.
SSID is just wireless name..
And network key means.. the wireless password or the WPS key.. I am not sure which terminology is which.
You must know the ssid and wireless security password..
But all devices not made by apple want names that are SMB compatible.. that means short no spaces and pure alphanumeric. Do not use apple names like
Fred Blog's Time Capsule Set 1
It is too long, it has spaces and it has an apostrople.
FBTC1 is fine.