MultiCharts has functions to draw trendlines and arrows on a chart.
But functions to draw rectangles are missing.
The enclosed package provides for 18 simple functions which allow drawing, modifying and deleting rectangles, with a syntax similar to trendlines (TL_*).
Details are given in the enclosed PDF document.
UPDATE:
a) This implementation does not allow filling the rectangle with color.
b) In this implementation, rectangle are internally defined as a set of 4 MultiCharts' segments (TL), but the user does not have to bother with it.
c) Two other ways to implement rectangles are referred at the end of the PDF document.