The usb webcam preview() function uses the USB Webcam support package, but not the Image Acquisition toolbox.
You can call upon preview() in your code from a GUI callback.
If you want the preview to appear in a specific place, then there is a way to do it that appears to be undocumented. First, create an axes of the appropriate size and position -- the preview has to be inside an axes. Secondly, create an image() of the appropriate size within that axes:
fig = figure();
ax = axes('Parent', fig, 'Units', 'normalized', 'Position', [0 0 1 1]);
cam = webcam(1);
camsize = str2double( strsplit(cam.Resolution, 'x'));
im = image(zeros(camsize), 'Parent', ax);
Everything above except the appropriate image sizing could be done ahead of time in GUIDE and saved.
With those done, we get to the undocumented part that needs to be done at run-time:
This tells preview to send the content updates to the given image object.