The ornamental design for a massageboard, as shown and described.
FIG. 1 is a top, perspective view of a massage board;
FIG. 2 is a bottom, perspective view thereof;
FIG. 3 is a left elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a back elevation view thereof;
FIG. 6 is a front elevation view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is an in use view thereof.
The surface of FIGS. 1-9 are lightly lined for contour only. The broken lines in FIG. 9 showing the massage board in use illustrate the environment and form no part of the claimed design.