MuseScore 3.5 is not available on the PortableApps.com site & Platform application
Workaround: As of version 3.5, MuseScore is available as a Windows portable application/installer (lower-case intentional, to distinguish it from an "official" PortableApp.com application), available from MuseScore downloads (link text: Portable version).
The effort to create the portable application binary/installer was tracked at #281616: MuseScore 3 is not available as a PortableApp on Windows may have some additional background.
Would it be possible to do the the necessary to have it conform to the PortableApps.com standards so it can be included?
(a) on the PortableApps.com site
(b) in the PortableApps.com launcher