The version of Multisim Spice offered by Analog Devices here does not include a device model for the TL431 group of voltage regulators, so when I needed one, I made one.
Going for the super-simple and idealized approach, all it took to make the TL431 model was a 2.495 volt DC source, a 3-terminal ideal op-amp and a voltage to current converter. The sketch below shows how that came out and how it worked.
Since this model is made of idealized elements, the load regulation seen in the above example is also ideal. However, this simplification was just fine for my purposes at the time.