coordinate system - How to apply a default datum transformation in an expression in QGIS

I'm using the expression:

x(transform($geometry, 'EPSG:32632', 'EPSG:3003'))

to add coordinates fields according to different CRS in a point layer. To obtain better accuracy i need to apply a Datum transformation (epsg:1662) in the CRS conversion. I have set the option "ask for default datum trasformation if several are avalaible" in Settings > Options that it should allow to select the datum transformation but using the expression in the field calculator this function does not seem to be available (the window does not appear).

How can I overcome this problem if possible or it would be better to create a custom CRS (how?) appplying the datum transformation to insert in the expression?