Bass clef to loud and treble clef not loud enough.

Attach a sample score with the problem.

MuseScore uses MIDI internally, probably what you saw applies to you. Anyhow, if you literally want to pl;ace different dynamic markings on the staves to tell a real pianist to do something other than play the hands equally loudly except with a slight emphasis on the melody as per common practice, then add the dynamics to each staff separately and use the Inspector to tell MuseScore to apply them to that staff only rather than the whole part. If you really want to have a single dynamic but just want to tweak the computer-generated playback for a little more realism by having MuseScore imitate the way a human musician would give a little extra emphasis to the melody, then select the melody notes, hit the Notes button in the Inspector, and add in a little offset to the "Velocity" setting.