how to insert rectangle to a video preview

im using my webcam to take an image of the user. I want to insert a rectangle to the video preview so that user can put his face within that boundary. I'm using the following block of code. But not getting any rectangle on video preview.

 cem=webcam('Lenovo EasyCam');
 preview(cem);
 pause();
 img=snapshot(cem);
 hold on 
 thisBB = [10 10 300 300];
 rectangle('Position',[thisBB(1),thisBB(2),thisBB(3),thisBB(4)],'EdgeColor','r','LineWidth',2 );