Tap it at the lamp
For the lamp, you need switched-hot and neutral.
The designers of this 3-way circuit wired it as a switch loop without neutral. Neutral cannot be found anywhere in the 3-way network, so you have no option to tap there.
You are not allowed to tap one wire in one place and another wire in another place. That would make currents unequal in cables (causing undesired cable heating, vibration and EMF) and risk overloading tapped neutrals (neutrals don't have breakers).
Therefore the only place you can tap to add lights in this lamp arrangement is the existing lamp.
If you want the new light to be separately switched, the answer remains the same, just in that case you want always-hot and neutral.