Hi again Chan, my dear friend! Practice the song as written, given the tempo you hear from the piano video. The grace note can be added later, once you are comfortable with the piece. The piece is like a yummy cake. It’s good enough alone. Sometimes you get lucky and there are sprinkles. 😀 The grace notes are sprinkles. They are not critical but they can make it more interesting.
Technically, in this piece, the grace note is added in the triplet figure in line 3, measure 4. Instead of playing second finger C natural first, add a quick note just before both C naturals. That note will be one step higher than the written melody, D natural, played with 3rd finger. It is done twice in this measure. You can hear it on the piano video. She also adds a note before C natural in the triplet at the end of line 4. This is also 3rd finger, played quickly before the written C natural.