While it varies slightly depending on the specific site, there is usually an option under the video you want to add that allows you to add it directly to a queue or playlist. However, you need to have an account with that service and be logged in at the time to add videos to a queue successfully. Some sites, such as YouTube, allow you to have multiple playlists or queues under the same account.
For Netflix, add a title by clicking on it and then selecting Add to My List. Netflix now allows up to five profiles per account, giving you five different playlists to customize for different genres or for other members in your household.
Hulu lets you add videos to a queue by hovering the cursor over the video and then selecting Add to Queue when it appears. If you select a TV show as a favorite, Hulu adds new episodes of that show to your queue automatically when they become available.
YouTube's specific method depends on the device you are using, but on the computer, you can find an Add to button under the video. Once you click on that, select Create New Playlist. Enter the name for your queue, and click Create. Subsequent videos can be added to that queue using the Add To button for that video.