Here's an example of the process with YouTube, others should be similar but may have different features and ways of getting the video URL:
On YouTube, find the video you want to post and go to the 'Share' tab (blue circle in the image) under the video frame:
Copy and paste the video URL (highlighted yellow in the image):
Click on the checkbox (red circle in the image) and enter a time if you want the video to start at a particular time (remember to use a semi-colon ':' to separate the minutes and seconds):
When posting a message as usual on the OSS (temp) forum, click on the 'Insert Video' button -
Then paste the address into the 'Video URL' field (paste here):
You can also post a direct link to a video file (see the types in the above image) that you might have stored on your Dropbox folder. In that case, just select the 'Copy public link' of the file by right clicking it and going to the 'Dropbox' sub-menu.
If you have any other questions about posting videos, please feel free to PM me and I'll try and help you.