The Draw tool icon, representing a human hand with a pencil in it, allows you to draw straight or freehand lines, similar to having a marker.
To draw a straight line:
Select the "Draw>Straight Line" tool.
Click and drag on the Whiteboard page to draw a line.
Release the mouse to end the line.
To draw a freehand line:
Select the "Draw>Freehand Line" tool.
Click and drag on the Whiteboard to draw a freehand line.
Release the mouse to end the line.