In piano scores, it is common to write a musical phrase extending across both staves—bass and treble. This can be entered in MuseScore as follows:
Enter the notation in one staff to begin with. e.g.
Select a note and press Ctrl+Shift + ↓/↑ (Mac: ⌘+Shift + ↓/↑). This moves all the notes in that voice down/up to the other staff. e.g.
Note : If you only want to move certain notes in a chord you need to ensure that they are in a separate voice.
To adjust the beam, double-click it to show the handles. Use the keyboard arrows or drag the handles to change the beam angle and position: