TSL: How do I set a `Vector3` attribute on a geometry and access it from within a TSL node?

Let’s say I have 10 geometries, and each geometry has a single Vector3 value associated with it called velocity. Each geometry has a different velocity value.

I also have a MeshBasicNodeMaterial with some TSL nodes that intend to access the velocity value for whichever geometry it is applied to.

How do I set an attribute on each geometry called velocity and access it in a TSL node?

Is an attribute even recommended for this?