1- Create a free account with YouTube and upload your video.

2- Log Into your Usefreelancer.com account and start adding your proposal and when you come to the part to add your video do the following

3- Brows your video on YouTube and the click the Share link under your Video.

4- Click Embed (its icon look like this <> 

5- Click Copy at the bottom of the Embed window or copy the code and

6- Past it inside the video box of your proposal