The ornamental design for a mainspowerplug, as shown and described.
FIG. 1 is a top view of the power plug illustrating the new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof; and,
FIG. 5 is a top perspective view thereof.
The left side is identical to the right side. The broken line feature is presented for illustrative purposes only and forms no part of the claimed design.