I am newbie in grasshopper, I want to elevate this surface along this curve. (shown in reference image)
take the opposite edge, loft
Congrats, you were already on the right track!
You can get the opposite curve to your moved curve by subtracting the index of latter in the edge list, from the total amount of edges (i.e. 4). This works for your example, since your dealing with a simple square/rectangle! You have selected the curve at index 3. 3 subtracted from 4 gives you index 1, which is the opposite curve for a square/rectangle/polygon with 4 sides.
Since the curves are all directed to flow along the border of the surfaces, the opposite curve also has the opposite direction of your initial curve. This would produce a twisted loft, which you probably don’t want. In order to remedy this, you simply flip the curve direction to match the direction of the initial curve, and your ready to loft both curves.