HELP！How to get flux of a face defined by my self?
 

I want to get flux of a little face defined by my self which Inside my model.
And i want to use swak4foam to do this.
But there is a question that i don't know how to define a surface in swak4foam.
I used to define a surface by basepoint and normal&vector in swak4foam,but i don't want such a surface cross the entire model. I just need a little rectangular section inside the model.

so now i want to know how define a little rectangular section surface in swak4foam.
or is there any other method to get the flux of a little rectangular section?