Using python 3 to fill in text boxes on websites, and clicking button to download

I want to download youtube videos with python. I found a website (http://ift.tt/2nBqICK) wich can download youtube videos in mp4 format.

I am using python 3, BeautifulSoup and requests to get the videos URL from youtube and I have no idea how can I download it from the website.

Is tehere any way to fill in the text box with the youtube video URL and then click the "Download MP4" button?