How to hear the harmonic while writing it normally ?

No you can't do that automatically. I'm guessing you want both proper notation and sound.

You have to notate it so it can be seen. Remove the check from play for these notes in the inspector (F8). Enter the notes you want to hear and make them invisible. I normally put the played note in an unused voice and then use the filter selector (F6) to select only that voice and make everything invisible at once.

I don't think it is actually possible to play a true harmonic unless the soundfont contained the sound of a violin harmonic. But if you can live with the correct pitch even if it hasn't the timbral quality, you could select the notes you don't want to sound, go to the inspector's category "Note" and uncheck Play. Then only the "harmonic" will sound (with the sound of a regular high-pitched violin note).