defining function depending on 3D spacial coordinate vector and time

Hello,

I am quite new to Comsol, and I would like to create a function depending on the 3D position
in the coordinate system of my geometry and the time. It would create a space and time dependent electrical pulse (function returning Volt). So, I would like to couple the geometrical
spatial position coordinate (x,y,z) and time (t) dependent electrical pulse as boundary condition to one of my terminals. It would try to simulate injection of electric pulse to a certain small part of a conductor
as a function of time.

I have naively tried to define functions with x,y,z, and t as arguments and gave it as a User Defined value for my terminal but I get error saying the x is not defined. Obviously, somehow I have to tell Comsol what x (and y, z and t) is.

Thanks for any help,
Balint