Adding coolant to a (potentially) very low 2nd gen

I have a 2008 Prius with 215k miles. I was never aware there were two coolant reservoir, and while the hybrid coolant is full, there's no coolant to be found in the engine's reservoir. I want to add coolant, but I'm unaware of the best way to do it.

1. Should I just fill the engine reservoir, wait and see if it goes low again, then add more? Or is it possible for me to add fluid directly to the radiator?

2. (Ignore question if I can't add directly to radiator) If I can add directly to the radiator, how much should I add? Should I add coolant until it nearly overflows? Or is there a better way to tell if the radiator is nearly full?

I'm just looking for tips on solving my low coolant situation here, thank you