how to manually trigger an image taken from usb webcam?

we take real time images from usb webcam. for this we have installed our usb webcam in matlab. but the problem is when we run the code we get snapshot of image immediately. Instead of this we want to take image after some delay(like manual triggering). here is code for taking image from webcam installed by me.

vid=videoinput('winvideo',2,'YUY2_640x480'); preview(vid); start(vid); set(vid,'ReturnedColorSpace','RGB'); rgb=getsnapshot(vid); imshow(rgb); stop(vid);