How to give Drain Voltage Of N-Channel MOSFET as an input to a Physical Model?

I am using Comsol 5.4. I am working on the physical model of Resistive RAM. I have to connect bottom terminal of model to the drain of n-channel MOSFET which I am taking from Electrical circuit module. My present approach is to make bottom terminal into voltage type and inputing the voltage from drain end of MOSFET. How do I do that? What variable should I use??