Integration coupling variables

I am solving the Navier-Stokes equation in 3D (time-dependent). Afterwards I like to compute the total kinetic energy by integrating the velocity squared over the volume. I like to have this energy available as a variable (so for example E(t); Energy as a function of time). In the Comsol documentation I read that this can be done using integration coupling variable (that integrates an expression over selected geometric entities). So in the model builder I right-click Definitions and then choose via Model Couplings, Integration. In the settings window I can then give the integration operator a name (let's say I call it E for energy) and I select the source (the 3d domain). But I do not see where I can specify the expression that needs to be integrated. So can anyone tell me where to specify the expression that the integration operator needt to act upon?

Thanks in advance,

Leon