First create fretboard diagrams for the chords in the first bar, and then drag the diagrams where you want them to be on your page.
Then add labels for them, for example as chord symbols. Select a note or rest and press Cmd-K (maybe Ctrl-K on Windows), type the chord's name and drag it where you want.
This is MuseScore 2.x. It might be different in 3.x.