On a Piano Stave we set from Format/Style/Page the Grand Staff distance to 0,2sp
From the Breaks and Spacers palette we add a Staff spacer fixed down at the bottom of the top Staff. From the Inspector set it to a height of 2 sp.
From the Add menu insert an horizontal frame to which add the text using the General Palette which is accessed by pressing F2, tab Musical Symbols->Clefs->C Clef
We adjust the position of the inserted text. The spacing adopted makes the C4 of the two Staves coincide.
As an alternative to the frame you can add an image (like the following) to be positioned
(right-click->save target as)
Example score: SolMiTest_0.mscz