Is this what you're after?

http://www.comsol.com/blogs/equation-based-modeling-in-comsol-4/

Hello Mr Banik,

If this tutorial could help you,

You can implement any kind of differential equation and set up the boundaries of your model by using PDE's interface,

You could read either this post

https://www.comsol.com/blogs/3-examples-of-equation-based-modeling-in-comsol-multiphysics/

or visualize this video for further explaination

https://www.comsol.com/video/equation-based-modeling-custom-simulations-comsol-multiphysics