Hi Unaz,
Yes, this is simple. Under the "Draw" menu use the "Line" tool to draw a closed path. By default, HFSS will fill the path to make a surface using the "CoverLines" process. Simply delete this, and you have your closed loop.
Then, use the fields calculator ("HFSS"->"Fields"->"Calculator"). Select "Quantity"->"H", then "Complex"->"Real" (or imaginary, unfortunately you can't leave it complex), then "Geometry..."->"Line" and select the line you drew. Press "Tangent", then the integrate symbol. Press "Add" and give the quantity a name.
To plot this value, create a new "Fields" report, and select your quantity under "Calculator Expressions".
Good Luck!