Adding Synth/Mixer on Musescore (internet)

Hello, Everybody!
I just uploaded a score to MuseScore and I want to be able to edit the different levels of sound on the MuseScore website. On other scores I've looked at, they have the "MuseScore Audio" and "Synthesizer" along with the option to enable it. How can I add that setting to the online version of my score? Do I have to add something to the score before uploading?
Let me know ASAP or if this question doesn't make sense. I'll see if I can upload a photo. Thanks a bunch! :)