You can add a ButtonPushedFcn callback for button and add your code for viewing video from a USB camera
For your reference, You can provide the below code in the callback function. You will be able to play the video by pushing the button.
videoFReader = vision.VideoFileReader('viplanedeparture.mp4');
videoPlayer = vision.VideoPlayer;
while ~isDone(videoFReader)
frame = step(videoFReader);
step(videoPlayer,frame);
end
pause(100);