How do I set the "debug object" for blueprint debugging?

Let’s say I have blueprint B that derives from blueprint A. In blueprint A, I have a function DoStuffs. From blueprint B, I call that DoStuffs function. When debugging that function, how do I specify that I want to debug an instance of B ? The drop down menu only shows “No debug object selected”. The tooltip says that I can set it in the level. I’ve searched for where to set it, but couldn’t find it anywhere.

Thanks!