Melody tied to chord

Enter all the notes.
Select the highest note in the triplet, and click T. Since there are no intervening notes, you get the tie. Simple.
Next select the middle note in the triplet, and CMD-click (on Mac, CTRL-click on Windows) the middle note on the chord, and click T. You may have to flip the tie (X) and fool with the control points.
Repeat for the lowest note.

Finicky, but it works.

Enter the triplet and the chord. Then click on each note of the triplet in turn and press T.

See https://handbook.musescore.org/basics/entering-notes-and-rests and scroll a long way down to see the section on ties and be sure to read the footnote.