  Recent observations suggest that planets formation starts early, in
protostellar disks of $\le10^5$ yrs, which are characterized by strong
interactions with the environment, e.g., through accretion streamers and
molecular outflows. To investigate the impact of such phenomena on disk
physical and chemical properties it is key to understand what chemistry planets
inherit from their natal environment. In the context of the ALMA Large Program
Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like
protostars (FAUST), we present observations on scales from ~1500 au to ~60 au
of H$_2$CO, HDCO, and D$_2$CO towards the young planet-forming disk IRS~63.
H$_2$CO probes the gas in the disk as well as in a large scale streamer (~1500
au) impacting onto the South-East (SE) disk side. We detect for the first time
deuterated formaldehyde, HDCO and D$_2$CO, in a planet-forming disk, and HDCO
in the streamer that is feeding it. This allows us to estimate the deuterium
fractionation of H$_2$CO in the disk: [HDCO]/[H$_2$CO]$\sim0.1-0.3$ and
[D$_2$CO]/[H$_2$CO]$\sim0.1$. Interestingly, while HDCO follows the H$_2$CO
distribution in the disk and in the streamer, the distribution of D$_2$CO is
highly asymmetric, with a peak of the emission (and [D]/[H] ratio) in the SE
disk side, where the streamer crashes onto the disk. In addition, D$_2$CO is
detected in two spots along the blue- and red-shifted outflow. This suggests
that: (i) in the disk, HDCO formation is dominated by gas-phase reactions
similarly to H$_2$CO, while (ii) D$_2$CO was mainly formed on the grain mantles
during the prestellar phase and/or in the disk itself, and is at present
released in the gas-phase in the shocks driven by the streamer and the outflow.
These findings testify on the key role of streamers in the build-up of the disk
both concerning the final mass available for planet formation and its chemical
composition.
