Hi, and welcome.
The Bassman 100 is designed to work with a 4 ohm speaker load. I have never ever seen one designed to work with an 8 ohm load, so that pic above is highly unusual.
Anyway, generally Bassman 100's (and 50's) would usually like to see a 4 ohm load.
The speaker outputs are wired in parallel, but you must connect a speaker to the "SPEAKER" output first, before connecting a second speaker to the "EXT. SPKR" output.
So, connecting an 8 ohm speaker to the first output, and a second 8 ohm speaker to the EXT output, would present a total load of 4 ohms to the amp, which is ideal.
Fenders can usually withstand a mismatch either way though, so you could easily run a single 8 ohm cab and the amp would be ok. Equally you could run two 4 ohm cabs, which would present a 2 ohm load to the amp, and it would be ok, but it might run a bit hotter.
I have a modified Bassman 50 which has a changed output transformer designed to see an 8 ohm load. I run it into a 4 ohm speaker without problems.
Rob