How to write down when one finger plays a long note on the piano, and the other fingers play short notes (sequentially quarter-notes) at this time.

Hi,
Writing down a piano score, faced a situation when first finger holds down one note (half-note), two other fingers at the same time press two quarter-notes one after another. The problem is that after written short notes, I cannot additionally insert a long one, i.e. MuseScore will either try to change shorter notes or make a chord.
Waltz in D minor from Tchaikovsky's "Children's Album" for flute and piano, piano part.
Any solution for that?
Thanks.