You can set any YouTube video to play in sync with a score, as an alternative to the default generated audio. You can find the “Manage audio sources” option behind the three dotted icon ⋮ next to the score title.
If you have already added any videos, they will be listed here; click on any of them to adjust the existing audio source, or click "Add an audio source" to add a new one.
Enter the URL of the video you want to add. Click “Save,” and you will be taken to the Audio source sync editor. It may take some time for the waveform to appear; wait until it does.
Usually, MuseScore.com's system is able to automatically sync the audio source with the sheet music, and measure markers will be placed at the appropriate points along the audio track. If it can't do it correctly, you can synchronize the video with your score manually.
You need to remember two keys: the space bar pauses and resumes playback, and the letter M on your keyboard places a marker and moves to the next measure. Essentially, press space to start playing the video, and then tap M on the downbeat of each measure.
When you’re done, there are checkboxes below that allow you to select whether you actually want the video to appear with the score, or only want the audio from it; and, if you have a Pro account, set this audio source to be the default. Anyone can still switch to one of the other available audio sources—when on the score page, click the ⋮ button in the top right corner of the score player (NOT the ⋮ button off to the right which only appears on your own scores) and click “Audio sources” in the dropdown menu.
See also Listen and view.